How to test an external SSD's transfer speed

Your external SSD may feel slower than expected, or you may want to check its speed before trusting it with a large transfer. A browser test can measure how quickly the connected drive reads and writes data, without installing a benchmarking app.

Use the USB & SD Speed Test to test the connection you actually use. The result reflects the drive, cable, port, any hub or adapter, and the test conditions together. It is not a guarantee that every file transfer will run at that speed.

Test the external SSD

1. Open the USB & SD Speed Test on the computer connected to the SSD. If the browser asks for access to files or a folder, choose the external drive or a folder on it.
2. Connect the SSD directly to the computer if possible. Wait until it appears in File Explorer on Windows or Finder on macOS, and close any file transfer already using the drive.
3. Choose a location on the SSD for the test if the tool asks. The test measures real read and write speeds, so it needs to write test data. Back up important files first, and do not choose an option to format or erase the drive.
4. Run the test and note both the read and write results, along with the units shown. Keep the same cable, port, and location for any repeat runs.
5. Run it two or three times after the first result. Similar results are a better indication of typical performance than one unusually fast or slow run.

For a useful comparison, do not copy files to or from the SSD during the test. Also pause cloud syncing or other large transfers if they are using the same drive or computer.

What the result means

Read and write speeds are both steady

The SSD and its current connection are delivering repeatable results for this test. Compare the numbers with the drive's stated speed only as a rough check. Advertised figures are often peak speeds measured under specific conditions, and a browser test may use a different workload.

Read speed is much higher than write speed

This can be normal. Some SSDs write more slowly than they read, especially during long transfers, when the drive is nearly full, or after a fast temporary write cache is used up. A browser test does not reproduce every file size or transfer pattern, so use it to compare the same drive under the same setup.

Both speeds are lower than expected

The slowest part may be the port, cable, adapter, hub, or drive. A USB-C connector by itself does not tell you the connection speed. Try a different port and a cable rated for data at the speed your drive supports.

Results change a lot between runs

Another task may be using the SSD, the computer may be busy, or the drive may slow down as it heats up. Close other transfers, let the drive cool, and repeat the test with a direct connection. A few small differences are normal, but large swings that continue across runs deserve more checking.

The browser cannot access the drive

The browser may not have permission to use the selected location, or the drive may not be mounted. Check that the SSD appears in the operating system, then try the browser's file or folder access prompt again. Do not grant access to a different drive by mistake.

How to improve a slow result

1. Connect the SSD directly. Remove the hub, dock, or adapter for a test, then connect the drive to another port. If this helps, the intermediary or its cable may be limiting speed.
2. Try the drive's data cable. Charge-only cables and older cables can fit the connector but transfer data more slowly. Use the cable supplied with the SSD or another cable explicitly rated for data transfer.
3. Stop other transfers. Pause copies, backups, downloads to the drive, and synchronization. Then run the test again without changing the connection.
4. Check that the drive is mounted and selected. On Windows 11, open File Explorer and select the external SSD under This PC. On macOS, open Finder and look for it under Locations. If it is missing, unplug and reconnect it, or try another port.
5. Compare another port or computer. Test the same SSD with the same cable on a different port, then on another computer if one is available. Change one part of the setup at a time so you can see what changes the result.
6. Check power and heat. A bus-powered SSD may behave poorly through an unpowered hub, and a hot drive may reduce its speed. Connect it directly, place it where air can reach it, and let it cool before testing again.

When the SSD or cable may be faulty

If the SSD repeatedly disconnects, disappears from the computer, or gives very low results through more than one known-good cable and direct port, test it on another computer before drawing a conclusion. Back up any files you can still read. A problem that follows the drive across computers may point to the SSD or its enclosure; a problem that follows one cable or port narrows the cause.

If the drive contains important data and starts disconnecting or producing read errors, stop running speed tests and make a backup rather than stressing it further. Contact the manufacturer about warranty service if the fault persists. A browser test can measure transfer performance, but it cannot diagnose internal drive damage or recover files.

Why the test may differ from copying a file

A speed test and a normal copy do not always use the same data size or pattern. One large file can transfer differently from thousands of small files, and a drive may write quickly at first before slowing during a longer transfer. The operating system can also cache data, meaning it temporarily keeps recently used information in memory.

Use the test to compare the same SSD, port, cable, and computer over time. It reads and writes test data on the selected drive in your browser, and your files stay on your device.