How to test a USB flash drive's real speed

Your flash drive says USB 3, but copying files still feels slow. Or you want to check a new or used drive before trusting it with important files. The number printed on the drive or its packaging does not tell you how fast it will perform in your computer.

The USB & SD Speed Test measures real read and write speed. Test the drive directly, then use the results and the checks below to work out whether the limit is the drive, the port, or the conditions of the test.

Test the USB flash drive

1. Open the USB & SD Speed Test and connect the flash drive to your computer. If possible, plug it directly into the computer rather than a hub or dock.
2. Follow the instructions on screen to select or access the drive. If your browser asks for access, choose the drive you want to test. Back up important files first, and leave enough free space for the test.
3. Run the write and read tests, and let each one finish. Do not unplug the drive or start copying other files while a test is running.
4. Write down both results in MB/s. If you are comparing ports or drives, repeat the test under similar conditions, using the same computer and browser.

A test measures the drive in the current setup, not just the flash memory. The port, any adapter or hub, background file transfers, and how full the drive is can all affect the result. A single run is useful, but a second run can help show whether an unusually low number was temporary.

What the result means

Read speed is much higher than write speed

This is common. Many flash drives can read files faster than they can save them, and advertised peak speeds often refer to reading rather than writing. If you mostly open files from the drive, read speed may matter more. If you regularly copy large videos or backups onto it, pay closer attention to write speed.

Both numbers are low

The drive may be connected through a slower port, hub, or adapter, or the drive itself may have modest performance. Try another direct USB port before deciding the drive is faulty. The USB connector's shape or color alone does not guarantee a particular speed.

The result changes a lot between runs

Another task may be using the drive, or the drive may slow down during longer writes as its cache fills. A cache is a small, faster area that can make short transfers look quicker than sustained transfers. Close file transfers and repeat the test after the drive is idle.

The result is lower than the number on the packaging

Packaging may list a peak speed measured under ideal conditions, not the speed you will get in every computer. The test reflects the connected drive and port at that moment. Compare the result with the drive's advertised read or write figure only if you are comparing the same type of speed.

The test cannot access the drive

The browser may not have permission to access it, or the drive may not be mounted by the operating system. Check that it appears in File Explorer on Windows or Finder on a Mac, then reconnect it and follow the browser's access prompt again.

How to fix it

1. Plug the drive directly into the computer. Remove hubs, docks, extension cables, and adapters for the next test. If the result improves, one of those connections may be limiting speed.
2. Try another USB port. A computer can have ports with different capabilities, even when the connectors look alike. On a desktop, try a port on the back of the computer as well as one on the front.
3. Stop other transfers. Pause file copies, backups, and downloads that use the drive or computer, then run the test again. If the drive is very warm, let it cool before another run.
4. Check which connection the computer sees. In Windows 11, right-click Start and open Device Manager, then expand Universal Serial Bus controllers. On macOS, open System Settings › General › About › System Report › USB. These screens can help you identify the connected device and bus, but the speed test is still the way to measure actual transfer performance.
5. Check free space and drive condition. A nearly full drive can behave differently from an empty one. Move files elsewhere before retesting, and do not format the drive unless you have copied anything you need to keep.
6. Test the drive on another computer. Use a direct port and repeat the same read and write test. If it remains unusually slow there, the drive is more likely to be the limit than your computer's port.
7. Eject the drive safely. Finish the test and wait for file activity to stop before disconnecting it. Use the eject option in File Explorer on Windows or the eject control in Finder on a Mac.

When the drive may be faulty

A low speed by itself does not prove that a flash drive is damaged. But repeated errors, disconnects, files that will not open, or speeds that collapse even on another computer are warning signs. Copy any readable files to a different device as soon as you can. If the drive is new, contact the seller or manufacturer about a replacement. Avoid relying on a drive that has already lost or corrupted files.

Why copying files can look different

The speed test reports read and write performance using its test data. Copying a folder of many small files can be slower than moving one large file because the computer has to handle each file and its folder information. Short transfers can also look faster because data may first be held in a cache. For a fair comparison, use the same port and conditions, and treat the test as a practical measurement rather than a promise of the speed of every file transfer.