Fix slow file copy speeds between drives on Windows 11

TL;DR

A slow copy is usually a disabled **write cache**, an antivirus scanning on-the-fly, a **USB selective suspend**, or the SMB signing default on network copies. Measure a 10 GB test file and fix the single likely cause.

Before you start

Requirements

Windows 11 25H2 on both source and destination
A single large test file (one 10 GB file is more useful than many small ones)
Admin rights

Step by step

Instructions

Create a repeatable baseline copy

Make a single ~10 GB test file (video, ISO or disk image). Copy it from source to destination with File Explorer and note the sustained MB/s shown in the dialog's chart. One large file isolates throughput; many small files measure overhead instead.

fsutil file createnew C:\test10gb.bin 10737418240

tipfsutil creates a sparse file instantly - good for read tests but not for write tests. For a real write test, copy a real existing 10 GB media file.

Check both drives' health and type

Open **Settings → System → Storage → Advanced storage settings → Disks & volumes** and confirm the media type (SSD or HDD) and the bus (NVMe, SATA, USB). Expectations: NVMe-NVMe 1-3 GB/s, SATA SSD 400-500 MB/s, HDD 100-180 MB/s, USB 3.2 Gen 1 around 100 MB/s.

Confirm write caching on the destination

Open **Device Manager → Disk drives**, right-click the destination drive, open **Properties → Policies**. Select **Better performance** and tick **Enable write caching on the device**. Reboot if the setting was changed.

⚠ warningOnly enable the aggressive 'Turn off Windows write-cache buffer flushing' option on a desktop with a UPS. On a laptop or without battery backup, data loss on power cut is a real risk.

Add a temporary Defender exclusion for the test

Real-time scanning throttles sustained writes. Add a short-term exclusion on the destination folder, repeat the copy, and compare. If the number jumps, the slowdown is Defender inspecting the stream.

⚠ warningRemove the exclusion when the test is done. Leaving a general destination folder excluded weakens protection.

Disable USB selective suspend for external drives

Open **Control Panel → Power Options → Change plan settings → Change advanced power settings → USB settings → USB selective suspend setting** and set it to **Disabled** for your active plan. This stops Windows from throttling USB-attached SSDs and HDDs during long copies.

For network copies, check SMB signing and compression

On the destination share (if you own it), enable SMB compression for pre-compressed-type files. On both machines, confirm you are on a wired or Wi-Fi 6/6E/7 link. SMB signing is required on Windows 11 25H2 for client connections and does add overhead - leave it on and upgrade the link instead.

Set-SmbServerConfiguration -EnableSMB2Protocol $true -EncryptData $false -Confirm:$false

Re-copy and compare

Repeat the 10 GB copy and compare MB/s to the baseline. If speed is within range of the slower drive's spec, you are done. If it is still off, swap cables or ports as a last variable - a worn USB-C cable can halve throughput.

Troubleshooting

Common issues

Copy starts fast then drops to 50 MB/s

You hit the SLC cache on a QLC SSD. There is no fix in software - the drive sustains its true QLC speed after the cache fills. Leave 20 percent free to keep the cache available.

File Explorer shows the progress dialog but no MB/s chart

Click 'More details' in the copy dialog to expand it. The chart is hidden by default on fresh profiles.

USB drive disconnects during long copy

Disable USB selective suspend (step 5) and try a rear motherboard port instead of a front panel port - front panel wiring often has worse signal integrity.

FAQ

Frequently asked

Is robocopy faster than File Explorer?

Not for a single file. For many small files, robocopy with /MT:16 is faster because it copies in parallel.

Does the file system matter?

NTFS is well-tuned for large copies. ReFS and exFAT are slower on small files. FAT32 caps individual files at 4 GB.

Should I defrag an HDD to speed up copies?

Yes for HDDs if the Optimize Drives tool reports high fragmentation. Never defrag an SSD.

How to undo this

Remove any Defender exclusion you added in Virus & threat protection → Exclusions. Re-enable USB selective suspend in Control Panel → Power Options if your workflow needs the power saving on battery. Device Manager → Disk drives policies can be returned to Better performance with 'Turn off Windows write-cache buffer flushing' unchecked, which is the safe default.

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