When to favor sustained speed over peak burst performance

A USB drive can start a transfer quickly, then slow down after a few seconds. That does not always mean it is faulty. The first fast burst may come from a temporary cache, while the slower rate is closer to what the drive can maintain.

For a few small files, the burst may be all you need. For a long video, a large backup, or repeated writes, steady speed matters more. The USB & SD Speed Test checks real read and write speed so you can compare a drive with the kind of work you do.

Compare the drive's read and write speed

1. Open the USB & SD Speed Test and follow its instructions to test the removable drive. Back up important files first, and check the on-screen prompt before allowing the test to write data.
2. Test the drive in the same port and reader you normally use. Connect it directly to the computer rather than through a hub, if possible.
3. Note both the read and write results. Read speed matters when opening or copying files from the drive. Write speed matters when saving files to it.
4. Repeat the test under the same conditions. If the result changes, close other programs using the drive and check that it has enough free space, then compare again.

Use the result as a comparison, not a guarantee for every file or device. The speed you see can also depend on the USB port, card reader, computer, and the drive's free space.

What the result means

The speed starts high, then drops during a long transfer

The drive may have a fast temporary cache that fills during a large write. Once it fills, the drive writes directly to slower storage. This pattern matters if you regularly copy files larger than the cache or write many files one after another. A short burst is less important for occasional small transfers.

The speed is lower but stays steady

A steady rate can be a better fit for a long task than a high initial peak followed by a sharp drop. Compare the sustained write rate with what your device needs. For video, convert the recording bitrate from megabits per second to megabytes per second by dividing by eight, then leave extra headroom for overhead and changes in the workload.

Read speed is much higher than write speed

This is common. Many flash drives and memory cards read files faster than they can save them. If you mainly view or copy files off the drive, prioritize read speed. If you record video, save backups, or regularly add large files, focus on write speed instead.

Results vary between repeats

Another process may be using the drive, the device may be warm, or a hub or card reader may be limiting the connection. A browser test measures the whole path from the computer to the storage, not the drive in isolation. Repeat the test using the same port and reader before deciding the drive is inconsistent.

How to get a fairer comparison

1. Check free space. A nearly full drive may behave differently from one with room available. On Windows 11, open Settings › System › Storage. On macOS, open System Settings › General › Storage. Do not delete files you need just to improve a test result.
2. Use the same connection. Test each drive in the same computer port and card reader. A slow reader or hub can hide the difference between two fast cards.
3. Pause other transfers. Close file copies, backups, and apps that may be reading or writing to the device. Wait for background activity to stop before testing again.
4. Compare drives at a similar fill level. Some drives slow down as they fill, so a nearly empty device and a nearly full one are not a clean comparison.
5. Test the job you care about. For regular large writes, pay attention to write performance over time. For loading files, focus on read performance. If the drive is for a camera, also check the camera maker's required card rating, since a browser test cannot certify that the card will meet every recording mode's needs.

When sustained speed matters most

Favor steady performance when each transfer is large enough to outlast a brief burst, when you write repeatedly, or when the device must keep accepting data without pauses. Examples include copying a large video archive, running a backup, recording long high-bitrate video, or saving repeated batches of photos.

Peak speed is more relevant for brief tasks, such as moving a handful of small documents or opening a file occasionally. Even then, very small files may feel slow despite a high peak rate because each file takes time to locate and open. A sequential speed test does not measure every part of that experience.

For recording, compare the card or drive's sustained write capability with the recording device's stated requirement, not just its advertised maximum speed. A video mode may produce data continuously, and a brief fast burst cannot make up for a write rate that falls below the data stream for too long.

Why advertised peak speed can be misleading

A peak figure describes a best-case result, often under specific conditions. The drive may reach it only for a short period, with a particular reader and a large amount of free space. Sustained speed describes performance after those favorable conditions change. Both numbers can be useful, but they answer different questions.

Use the same setup for comparisons and focus on the part of the result that matches your task. If a device must record continuously, a stable write rate is more useful than an impressive starting burst.