New USB drive writing slowly? How to check

A new USB drive can seem fast for a few seconds, then slow down while copying a large folder. Or it may write slowly from the start. Before returning it, check the drive's actual write speed and rule out the port, adapter, and file size.

The USB & SD Speed Test measures real read and write speed in your browser. Compare the write result with the drive's stated speed, then use the checks below to find the likely cause.

Measure the drive's write speed

1. Back up important files on the drive, then open the USB & SD Speed Test. The test writes data to the drive, so do not test a drive that holds the only copy of important files.
2. Connect the drive directly to your computer if possible. Avoid a hub, dock, or extension cable for the first run.
3. Follow the tool's prompts to select the removable drive and allow access if your browser asks. Run the read and write measurements.
4. Note the write speed, then repeat the test once. Compare the results, and check the drive's packaging or product details for a write-speed figure, not just a read-speed figure.

Use the same computer and port for both runs. A result in MB/s means megabytes per second. Internet speed is usually shown in Mbps, megabits per second, which is a different unit.

What the result means

Write speed is low, but read speed is much higher

This can be normal. Some drives advertise a high maximum read speed but have a much lower write speed. Check the exact write specification for your model. A drive can also write quickly at first and slow down during a long transfer when its fast temporary cache fills.

Both read and write speeds are low

The connection may be limiting the drive. A USB 2 port, hub, adapter, or cable can prevent a faster drive from reaching its full speed. A slow computer or other activity using the same storage can also affect the measurement.

The result changes a lot between runs

Background activity, available space, and the drive's cache can make results vary. A short test may capture a fast burst rather than sustained performance. Repeat the test with other apps closed, then copy one large file to see whether the slowdown continues.

The speed stays far below the stated write speed

First check whether the label gives a maximum or a guaranteed sustained speed, and whether your connection supports the drive's interface. If the drive remains much slower after trying a direct port and another computer, it may be defective or not match its advertised specification.

How to fix slow writes

1. Use a direct USB port. Disconnect the drive from a hub or dock and plug it into the computer. Try another port. USB ports can look alike while supporting different speeds, and an adapter can limit the connection.
2. Close apps that are using the drive. Pause backups, file synchronization, antivirus scans, and large downloads while testing. On Windows 11, check Settings › System › Storage for storage activity and available space. On macOS, check Apple menu › System Settings › General › Storage.
3. Leave free space on the drive. A nearly full drive may write more slowly, especially during large copies. Move or remove files you no longer need, but keep a backup before deleting anything.
4. Try a large file instead of many tiny files. Small files take extra time to create and organize, so a folder with thousands of photos or documents can copy slowly even when the drive's large-file speed is reasonable. Compare with one large file of a similar total size.
5. Check the drive for errors. On Windows 11, open File Explorer, right-click the drive, choose Properties › Tools › Check, and follow the prompts. On macOS, open Disk Utility, select the drive, and choose First Aid. Back up important files before checking or repairing a drive.
6. Test on another computer. Use a direct port and repeat the same test. If the drive is slow on multiple computers while other drives work normally, the new drive is a stronger suspect.
7. Update the computer's USB drivers or system software. On Windows 11, check Settings › Windows Update and Device Manager › Universal Serial Bus controllers. On macOS, check System Settings › General › Software Update. Restart after installing updates.

When the drive may be faulty

A low result alone does not prove a new drive is defective. The product may list only its best-case read speed, or a short test may reflect a fast cache rather than sustained writing. Check the stated write speed and confirm that the port is not limiting it.

If repeated tests on two computers show very low writes, or copying stops, produces errors, or makes files unreadable, stop using the drive. Keep your receipt and test notes, and ask the seller for a replacement or refund while the return window is open. Do not rely on a questionable drive for backups.

Why a fast start can turn into a slow transfer

Many flash drives use a portion of their storage as a fast temporary writing area. Once that area fills, the drive may need to move data internally before it can continue at full speed. This is why a small test or a few small files can look fast while a long copy slows down. A sustained slowdown is not automatically a fault, but it should be consistent with the drive's stated performance.

The speed test runs in your browser and works with the drive connected to your device. Back up valuable files before any test that writes to removable storage.