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File Copy Freezes, Gets Stuck at 99%, or Copy Speed Drops to 0 Bytes/s in Windows 11/10 – Causes and Complete Fixes

Copying files is one of the most basic Windows operations, but sometimes a transfer that starts normally suddenly slows down, shows 0 bytes/s, remains at 99%...

Bison Technical Team Enterprise IT specialists

Updated 29 Aug 2026 19 min read 0 total views

Copying files is one of the most basic Windows operations, but sometimes a transfer that starts normally suddenly slows down, shows 0 bytes/s, remains at 99% for several minutes, or appears to freeze completely.

You may experience symptoms such as:

File copy remains at 99%.
Transfer speed suddenly falls to 0 bytes/s.
File Explorer stops responding while copying.
A transfer starts at 100–500 MB/s and suddenly falls to a few MB/s.
Copying to a USB drive pauses repeatedly.
An external hard drive becomes extremely slow during large transfers.
A transfer completes most of the data quickly but takes a long time to finish the final percentage.
Copying many small files is dramatically slower than copying one large file.
Network file transfers suddenly stop or fluctuate heavily.
Windows reports 100% disk usage even though the transfer appears frozen.

These symptoms do not always mean Windows is damaged. File-copy performance depends on the source drive, destination drive, file system, storage cache, USB/SATA/NVMe interface, antivirus scanning, disk health, file sizes, network connection, and other applications using the disks.

This guide explains how to identify the actual bottleneck and fix it.

1. First Understand Why Copy Speed Can Drop to Zero

A file copy operation involves more than simply moving data from one location to another.

Windows generally performs something similar to:

Source Storage → Read Data → Memory/Cache → Process Data → Destination Storage → Write Data → Update File System

If any part of this chain becomes temporarily busy, Windows may show:

Speed: 0 bytes/s

for a short period.

Therefore, a temporary drop to zero does not automatically indicate a fault.

It becomes a concern when the transfer remains at zero for a long time, File Explorer stops responding, the storage device disconnects, or Windows generates disk/file-system errors.

2. Wait a Few Minutes Before Cancelling a Transfer Stuck at 99%

When a file transfer reaches 99%, it can appear that practically everything has already been copied.

However, Windows may still be performing background operations such as:

Flushing cached data to the destination
Updating file-system metadata
Closing file handles
Writing directory information
Scanning files
Processing security information
Creating thumbnails
Updating timestamps
Finishing delayed writes

This is especially noticeable with:

USB flash drives
External hard drives
Large numbers of small files
Slow HDDs
Network drives
Older storage devices

If disk activity is still occurring, give the operation some time before forcing File Explorer to close.

3. Check Task Manager While the Copy Is Frozen

Task Manager can quickly reveal whether the transfer has actually stopped.

Press:

Ctrl + Shift + Esc

Open:

Performance

Check:

Disk

Observe:

Active time
Read speed
Write speed
Disk response
CPU usage
Memory usage

Also check the Processes tab and sort by Disk.

If the destination disk is still writing data even though File Explorer shows 99%, Windows may simply be finishing cached writes.

4. Determine Whether the Source or Destination Is the Bottleneck

A file transfer can only run as fast as the slowest component involved.

For example:

Fast NVMe SSD → Slow USB Flash Drive

The USB drive determines the actual sustained transfer speed.

Similarly:

Slow HDD → Fast NVMe SSD

The HDD becomes the bottleneck.

Test different combinations where possible.

For example:

SSD → SSD

SSD → USB

USB → SSD

HDD → SSD

This can help identify the problematic device.

5. Copy One Large File as a Test

Copying thousands of small files can be dramatically slower than copying one large file of the same total size.

For example:

One 10 GB file

may copy much faster than:

100,000 files totaling 10 GB

Every individual file requires additional operations involving:

File creation
File-system metadata
Permissions
Timestamps
Directory updates
Antivirus inspection

Therefore, test with a single large file.

If large files transfer normally but folders containing thousands of files are slow, the storage device may be working correctly.

6. Restart File Explorer

File Explorer itself can occasionally become unstable.

Open Task Manager:

Ctrl + Shift + Esc

Locate:

Windows Explorer

Right-click it and select:

Restart

Then try the transfer again.

Do not restart Explorer while an important transfer is actively writing unless you have already decided to cancel that transfer.

7. Restart the Computer

A restart can clear:

Stuck Explorer processes
Locked files
Pending storage operations
Driver problems
Memory pressure
Temporary Windows errors

After restarting, perform the copy before opening unnecessary applications.

8. Check Free Space on the Destination Drive

Make sure the destination has sufficient free storage.

Open:

This PC

Check the destination drive's available space.

Having exactly enough space for the files is not always ideal. Windows and applications may require additional temporary working space.

If the disk is almost completely full, free some storage before trying again.

9. Check for FAT32 File Size Limitation

This is particularly important for USB flash drives.

FAT32 cannot store a single file larger than approximately 4 GB.

To check the file system:

Right-click the destination drive → Properties

Look for:

File system

Possible values include:

NTFS
exFAT
FAT32

If you regularly need to transfer files larger than 4 GB, NTFS or exFAT may be more appropriate depending on how the device will be used.

Warning: Formatting a drive erases its contents. Back up important data before formatting.

10. Try Another USB Port

If the issue occurs with a USB drive or external storage device, connect it to another USB port.

Prefer a direct motherboard/system USB port rather than an unpowered USB hub.

A slow or problematic USB port can cause:

Transfer pauses
Low speed
Device resets
Disconnect/reconnect events
0 bytes/s transfers

11. Check Whether You Are Using USB 2.0 or USB 3.x

USB interface speed can significantly affect transfers.

USB 2.0 is considerably slower than modern USB 3.x interfaces.

However, remember that the USB standard is only the theoretical interface capability.

Actual speed also depends on:

Flash memory
SSD/HDD performance
Controller
File size
Number of files
Device temperature

Connecting a slow flash drive to a high-speed USB port will not magically make its internal flash memory faster.

12. Replace the USB Cable

For external HDDs and SSDs, the cable can cause surprisingly difficult-to-diagnose problems.

A damaged or poor-quality cable may result in:

Intermittent transfer freezes
Device disconnects
Very low speeds
Transfer retries
Drive disappearing temporarily

Try another known-good compatible cable.

This is especially useful when the same drive works normally on another computer.

13. Avoid USB Hubs During Troubleshooting

Connect the storage device directly to the computer.

Temporarily avoid:

USB hubs
Docking stations
USB extension cables
Front-panel extensions

This helps eliminate power and signal-quality issues.

14. Check the Drive for File-System Errors

Windows includes CHKDSK for checking file-system problems.

Open Command Prompt as Administrator.

Run:

chkdsk X: /scan

Replace X: with the actual drive letter.

For example:

chkdsk E: /scan

The /scan option performs an online scan on supported NTFS volumes.

If repairs are required, Windows may recommend additional CHKDSK options or scheduling a repair.

Important

Do not blindly run aggressive disk-repair operations on a drive that is physically failing and contains important data.

If a drive is clicking, disappearing, producing I/O errors, or showing serious SMART warnings, prioritize data recovery or backup before stressing it with extensive repair operations.

15. Check Drive Health

A failing HDD or SSD can cause file transfers to freeze while Windows repeatedly attempts to read or write problematic areas.

Warning signs include:

Copy freezes on specific files
Disk frequently reaches 100% active time
Extremely high response time
I/O errors
Drive disappears
Windows asks to scan/repair the drive repeatedly
HDD makes unusual mechanical noises
SMART monitoring reports warnings

You can perform a basic status check in PowerShell:

Get-PhysicalDisk

Review values such as:

HealthStatus

and

OperationalStatus

For more detailed diagnosis, use the SSD/HDD manufacturer's diagnostic utility where available.

16. Test Whether One Particular File Is Causing the Freeze

If the copy always freezes at approximately the same location, one file may be corrupted or unreadable.

Try copying folders separately.

For example, instead of copying:

D:\Backup

all at once, copy its subfolders individually.

Eventually, you may identify the file or folder responsible.

If Windows consistently cannot read one particular file from an HDD, investigate the source drive's health.

17. Temporarily Test Antivirus Impact

Security software may scan files during copy operations.

This can have a major performance impact when transferring thousands of:

EXE files
ZIP archives
Scripts
Documents
Email files
Small files

You can check Task Manager to see whether Microsoft Defender or another antivirus program is consuming significant CPU or disk resources during the transfer.

Do not permanently disable security protection merely to increase file-copy speed.

If you perform a temporary diagnostic test, restore protection immediately afterward and avoid testing with untrusted files.

18. Check for Other Programs Using the Disk

Another application may be heavily accessing the same disk.

Examples include:

Antivirus scans
Windows Search indexing
Windows Update
Backup software
Cloud synchronization
OneDrive
Virtual machines
Database applications
Torrent clients
Disk imaging software

Open Task Manager and sort processes by:

Disk

If another process is heavily using the source or destination drive, allow it to finish or safely close it before retrying the transfer.

19. Understand SSD Cache Exhaustion

This is a common reason for a transfer that starts extremely fast and then suddenly becomes much slower.

Many SSDs use a fast write cache.

For example, a transfer might initially show:

500 MB/s

and later fall significantly.

The initial data is being written into a high-speed cache. Once that cache fills, the SSD must write directly to slower underlying NAND storage.

This behavior can be particularly noticeable on:

Budget SSDs
DRAM-less SSDs
QLC SSDs
Nearly full SSDs
Large sustained transfers

Therefore:

Fast beginning + slower sustained speed

does not necessarily indicate a defective SSD.

20. USB Flash Drives Can Behave Similarly

Many USB flash drives have excellent advertised peak speeds but poor sustained write performance.

A drive may initially write at:

80 MB/s

and later drop to:

10 MB/s

or lower.

Low-cost flash drives may also pause periodically while managing internal flash memory.

Test another known-good high-performance USB drive before concluding that Windows is responsible.

21. Check Storage Temperature

Modern NVMe SSDs and some external SSDs can become very hot during sustained transfers.

When temperatures become excessive, the controller may reduce performance to protect the hardware.

This is called:

Thermal throttling

Typical symptoms include:

Transfer starts very fast.
Speed gradually decreases.
Performance becomes inconsistent.
Speed improves after the drive cools.

Ensure adequate system ventilation and avoid covering external SSDs during large transfers.

22. Update Windows

Install applicable Windows updates:

Settings → Windows Update → Check for updates

Windows updates can include reliability and compatibility fixes affecting:

Storage
USB
File Explorer
Networking
Device drivers

Restart after completing updates when required.

23. Check Storage and Chipset Drivers

Storage performance can also be affected by drivers.

Depending on the computer, relevant drivers may include:

Chipset
SATA/AHCI
NVMe
USB controller
Intel storage controller
AMD chipset/storage components

Prefer drivers supplied through Windows Update or the computer/motherboard/storage manufacturer's official support resources.

Avoid random third-party "driver updater" utilities.

24. Repair Windows System Files

If File Explorer or Windows storage components appear corrupted, run System File Checker.

Open Command Prompt as Administrator.

Run:

sfc /scannow

Allow it to complete.

If Windows reports component-store problems or SFC cannot repair files, run:

DISM /Online /Cleanup-Image /RestoreHealth

After DISM completes, run:

sfc /scannow

again.

Restart Windows afterward.

25. Use Robocopy for Large or Problematic Transfers

For large transfers, backups, or transfers involving many files, Robocopy can be more practical than File Explorer because it provides better control and logging.

Example:

robocopy "D:\Source" "E:\Destination" /E /Z /R:2 /W:2

Explanation:

/E – Copies subdirectories, including empty directories.

/Z – Uses restartable mode.

/R:2 – Retries failed copies twice.

/W:2 – Waits two seconds between retries.

For example:

robocopy "D:\Customer Data" "E:\Backup\Customer Data" /E /Z /R:2 /W:2

Robocopy is especially useful when File Explorer repeatedly freezes because of one problematic file.

26. Create a Robocopy Log

For troubleshooting, generate a log:

robocopy "D:\Source" "E:\Destination" /E /Z /R:2 /W:2 /LOG:"C:\copylog.txt"

After completion, examine:

C:\copylog.txt

The log can help identify files that failed or required retries.

27. Be Careful With Robocopy /MIR

You may see guides recommending:

/MIR

This option mirrors directories.

It can also delete files from the destination that do not exist in the source.

Do not use /MIR unless you specifically understand and require mirror synchronization.

For ordinary copying, /E is generally safer.

28. Network File Copy Freezes or Drops to Zero

If the problem occurs while copying files between computers, servers, NAS devices, or shared folders, the storage device may not be the only factor.

The path becomes:

Source Disk → Network Adapter → Network → Network Adapter → Destination Disk

Check:

Ethernet cable
Wi-Fi quality
Network switch
Router
NIC driver
Server load
NAS load
Destination disk performance

For troubleshooting, test using a wired Ethernet connection where possible.

29. Test Network Stability

Open Command Prompt and continuously ping the destination system:

ping -t 192.168.1.10

Replace the address with the correct server/NAS IP.

Look for:

Request timed out
High latency
Intermittent packet loss

Stop the test with:

Ctrl + C

Packet loss during file transfers can indicate a network problem rather than a Windows File Explorer problem.

30. Check Whether Wi-Fi Is the Bottleneck

Large file transfers over Wi-Fi may fluctuate significantly because of:

Signal interference
Distance
Congested channels
Power-saving behavior
Multiple connected clients
Mesh transitions
Weak access point signal

If possible, temporarily test the same transfer over Gigabit Ethernet.

If Ethernet works normally, troubleshoot the wireless network separately.

31. Do Not Force-Remove a Drive While It Is Stuck at 99%

This is important.

If Windows displays 99% but cached data is still being written, physically disconnecting the drive can cause:

File corruption
File-system corruption
Incomplete files
Lost data

Check Task Manager for disk activity before disconnecting the device.

When possible, cancel the transfer normally and use:

Safely Remove Hardware

before unplugging external storage.

32. File Explorer Says 0 Bytes/s but Disk Is Still Active

The copy dialog's speed indicator is not always a perfect representation of what the storage hardware is doing at every instant.

If File Explorer shows:

0 bytes/s

but Task Manager still shows disk write activity, allow the operation additional time.

Windows may be:

Flushing cache
Updating metadata
Processing delayed writes

33. Why Copying Many Small Files Is Slow

Consider two workloads:

Workload A

One file:

20 GB

Workload B

200,000 files totaling:

20 GB

Workload B can take dramatically longer.

The system must repeatedly:

Open a file.
Read it.
Create the destination file.
Write data.
Update metadata.
Close the file.
Repeat.

Antivirus software may also inspect each individual file.

This is normal storage behavior.

34. Compress Large Collections of Small Files

If you frequently transfer hundreds of thousands of small files, consider creating an archive first.

For example:

ProjectFiles.zip

You then transfer one large archive instead of thousands of individual files.

This can sometimes substantially improve transfer efficiency.

After copying, extract the archive at the destination.

35. Check Whether the Destination Is Almost Full

SSDs can lose write performance when very little free space remains.

If an SSD is almost completely full:

Write performance may decrease.
Garbage collection becomes more difficult.
Large sustained writes may fluctuate.

Freeing some storage can improve performance.

36. Test the Drive on Another Computer

For external storage, this is one of the most useful isolation tests.

Connect the drive to another computer and copy the same large test file.

If the same problem occurs

Suspect:

Drive
Cable
Enclosure
File system
Storage hardware

If the drive works normally

Investigate the original computer's:

USB controller
Drivers
Windows installation
Antivirus
Background applications
Hardware ports

37. Test With Another Drive

Perform the reverse test.

Connect another known-good USB drive, HDD, or SSD to the affected computer.

If every storage device freezes, investigate Windows, drivers, USB controllers, or the computer hardware.

If only one drive freezes, focus troubleshooting on that particular device.

38. Look for Disk Errors in Event Viewer

Event Viewer can reveal storage problems that File Explorer does not clearly display.

Press:

Win + R

Enter:

eventvwr.msc

Navigate to:

Windows Logs → System

Look around the time of the failed transfer for storage-related warnings or errors.

Possible sources may include:

Disk
Ntfs
storahci
stornvme
volmgr
USB-related components

Repeated disk I/O or controller errors should not be ignored.

39. Check Resource Monitor

For more detailed storage activity:

Press:

Win + R

Enter:

resmon

Open the:

Disk

tab.

Resource Monitor can show:

Processes using the disk
Files being accessed
Read activity
Write activity
Response time

This can help determine what Windows is doing when File Explorer appears frozen.

40. Clean Boot Windows for Troubleshooting

If the problem occurs with multiple drives and you suspect third-party software, perform a clean boot.

Press:

Win + R

Type:

msconfig

Under Services:

Select:

Hide all Microsoft services

Then disable unnecessary third-party services for testing.

Also review startup applications in Task Manager.

Restart and test again.

Remember to restore the required services afterward.

Recommended Troubleshooting Order

For most users, the following sequence is efficient:

Wait briefly if the transfer is stuck at 99%.
Check Task Manager for disk activity.
Restart Windows.
Check destination free space.
Try copying one large test file.
Try another USB port.
Replace the USB cable if applicable.
Avoid USB hubs.
Test the storage device on another computer.
Test another drive on the affected computer.
Check the file system.
Check disk health.
Run CHKDSK diagnostics where appropriate.
Check Event Viewer for disk/controller errors.
Check antivirus/background disk usage.
Update Windows and relevant drivers.
Run DISM and SFC if Windows corruption is suspected.
Use Robocopy for large or problematic transfers.

How to Interpret Common Symptoms

Copy starts fast and then becomes slower

Often caused by storage write-cache exhaustion.

Common with:

SSDs
USB flash drives
External SSDs

It may be normal.

Copy speed repeatedly drops to zero and recovers

Possible causes include:

Slow flash memory
Cache flushing
Background disk activity
Antivirus scanning
Thermal throttling
Storage controller behavior

Occasional brief pauses can be normal.

Copy freezes permanently on the same file

Investigate:

Corrupted file
Source disk problem
Bad sector
File-system corruption

External drive disappears during copying

Investigate:

USB cable
USB port
Power
Enclosure
Drive hardware

This is more serious than normal speed fluctuation.

File Explorer freezes but Robocopy works

The underlying storage may still be functional. Explorer itself, shell extensions, antivirus interaction, or handling of problematic files may be contributing to the issue.

All drives copy slowly

Investigate:

Windows
Drivers
Antivirus
CPU/RAM pressure
Storage controller
System-wide background processes

When Should You Suspect a Failing Drive?

Take the problem more seriously if you experience several of the following:

Frequent transfer freezes
Files becoming corrupted
I/O errors
Drive repeatedly disconnecting
SMART health warnings
HDD clicking or unusual noises
Windows repeatedly requesting disk repair
Extremely high response times
Files becoming unreadable
Transfer repeatedly failing at the same location

If important data is still accessible, back it up immediately.

Do not continue repeatedly stress-testing a drive that appears to be physically failing.

FAQ

1. Why does file copying get stuck at 99%?

Windows may still be flushing cached data, updating file-system metadata, closing files, or processing security and antivirus operations. A short delay at 99% can therefore be normal.

2. Why does file-copy speed suddenly become 0 bytes/s?

The destination storage may temporarily stop accepting new data while it processes cached writes, manages flash memory, performs internal maintenance, or handles another storage operation.

3. Is 0 bytes/s always a problem?

No. Brief drops can occur normally. A problem is more likely if the transfer remains at zero for a long period, the drive disappears, File Explorer permanently freezes, or disk errors are recorded.

4. Why does my SSD start fast and then slow down?

Many SSDs use a high-speed cache. Once the cache fills during a sustained write, speed can fall to the SSD's native sustained-write performance.

5. Why is my USB flash drive extremely slow?

Possible reasons include slow flash memory, USB 2.0 operation, small-file workloads, cache exhaustion, overheating, poor-quality hardware, or a failing flash drive.

6. Why is copying thousands of small files so slow?

Each file requires separate file-system operations and may be individually scanned by security software. Thousands of small files can therefore take much longer than one large file.

7. Can antivirus software slow file copying?

Yes. Real-time security scanning can increase transfer time, particularly when thousands of small files, scripts, executables, or archives are involved.

8. Should I permanently disable Microsoft Defender?

No. Permanently disabling security protection simply to improve file-copy performance is not recommended.

9. Can a bad USB cable cause file-copy freezing?

Yes. A damaged or poor-quality cable can cause errors, retries, disconnects, and unstable transfer speeds.

10. Can a bad hard drive cause copying to freeze?

Yes. Windows may repeatedly retry unreadable sectors, causing extremely slow transfers or apparent freezes.

11. How can I check whether my disk is failing?

Check SMART/health information, Windows Event Viewer, manufacturer diagnostics, and symptoms such as I/O errors, disappearing drives, corrupted files, or abnormal HDD noises.

12. Why does Task Manager show 100% disk usage during copying?

The drive can be fully occupied handling storage requests even when its MB/s throughput appears relatively low. This is common with HDDs and workloads containing many small files.

13. Why does the copy dialog show 0 bytes/s while Task Manager shows disk activity?

File Explorer's transfer indicator and actual hardware activity are not identical measurements. Windows may still be processing or flushing previously cached data.

14. Can low free space affect SSD copy speed?

Yes. SSD write performance can deteriorate when very little free space remains.

15. Why can't I copy a file larger than 4 GB to my USB drive?

The drive may use FAT32, which has an individual file-size limit of approximately 4 GB.

16. Should I format my USB drive as NTFS?

NTFS can be appropriate when primarily using the drive with Windows and when large-file support is required. Formatting deletes existing data, so back up the drive first.

17. Is exFAT suitable for large files?

Yes. exFAT supports files larger than 4 GB and is commonly used for removable storage requiring compatibility across multiple operating systems and devices.

18. Can overheating cause SSD transfer speeds to fall?

Yes. SSD controllers may reduce performance when temperatures become excessive.

19. Why does copying over Wi-Fi freeze?

Possible causes include weak signal, interference, packet loss, congestion, network adapter problems, or problems with the destination server/NAS.

20. Should I use Robocopy instead of File Explorer?

For large backups, thousands of files, restartable transfers, and troubleshooting failed copies, Robocopy can be more reliable and informative.

21. What does Robocopy /Z do?

/Z enables restartable mode, allowing interrupted file copies to resume more effectively.

22. Is Robocopy /MIR safe?

It must be used carefully. /MIR makes the destination mirror the source and can delete destination files that are absent from the source.

23. Should I disconnect a USB drive when copying is stuck at 99%?

Not immediately. The drive may still be writing cached data. Check disk activity and cancel the operation safely before disconnecting whenever possible.

24. Why does copying always freeze on one specific file?

That file may be corrupted, locked, unreadable, or stored in a damaged area of the source disk.

25. Will SFC fix file-copy problems?

SFC can repair corrupted Windows system files but will not repair defective storage hardware or damaged user files.

26. What does DISM /RestoreHealth do?

DISM repairs the Windows component store used to maintain and repair Windows system files.

27. Can Windows Search indexing slow a large file transfer?

It can contribute to additional disk activity, particularly when many newly created files are being processed.

28. Can cloud synchronization interfere with copying?

Yes. OneDrive or another synchronization application may immediately begin processing files written into a synchronized folder, increasing disk and network activity.

29. Why is an external HDD slower than an SSD?

Mechanical hard drives rely on physical spinning platters and moving heads. They generally have much higher access latency and lower random-performance capability than SSDs.

30. What should I do first if an important drive starts freezing?

Prioritize copying irreplaceable data to another healthy storage device. If the drive shows signs of physical failure, avoid unnecessary repair scans or repeated stress tests before securing the data.

Conclusion

A Windows file transfer that freezes, remains at 99%, or drops to 0 bytes/s does not automatically mean Windows is corrupted.

The most common causes include:

Storage write-cache behavior
Slow USB flash memory
Large numbers of small files
Antivirus scanning
Insufficient free space
USB cable or port problems
SSD thermal throttling
File-system errors
Corrupted source files
Failing HDD/SSD hardware
Network instability
Storage driver problems
Background disk activity

The key is to determine whether the bottleneck is the source drive, destination drive, USB/network connection, Windows itself, or a background application.

If the transfer merely pauses briefly and resumes, the behavior may be normal. If the drive disconnects, files become corrupted, Windows records repeated disk errors, or transfers consistently fail at the same location, investigate the storage hardware immediately and protect important data before performing extensive repair operations.