How to check whether an SD card handles sustained writes

Your SD card may start recording quickly but slow down during a long video, or a file transfer may begin fast and then crawl. A short speed check can show whether write performance is consistent, but it cannot always prove that a card will hold its speed for a long recording.

Use the USB & SD Speed Test to measure real write and read speed, then repeat the test and compare the results. This guide explains what those numbers can tell you, how to separate a card issue from a reader bottleneck, and when to test the card in the device you plan to use.

Check the card's write speed

1. Back up important photos and files before testing. Insert the SD card into a reader connected to your computer, and close any app that is transferring files to or from it.
2. Open the USB & SD Speed Test. Follow the browser's prompt to select the SD card or its test location. Grant access only to the card you intend to test.
3. Run the write-speed measurement, then note the result and its units. If the tool also shows read speed, record that separately. Write speed is the figure that matters for recording video or saving bursts of photos.
4. Run the write measurement again under the same conditions. If you can, repeat it a few times, with a short pause between runs. Look for a stable pattern, not just the highest number.
5. If the result is much lower than expected, repeat the test with the card in a different reader or USB port. Keep the card and computer the same so you can see whether the reader or connection changes the result.

A browser speed test measures a real transfer to the selected storage, but the result can be affected by the card reader, port, computer load, and the size and duration of the test. Treat it as a useful check, not as a full certification of long-term recording performance.

What the result means

The write speed stays similar across runs

Consistent results suggest that the card and reader are behaving steadily during the test. They do not prove that the card will sustain that rate through a long recording, especially if the test is shorter than your real workload. For a camera or recorder, make a test recording at the intended resolution and frame rate, then play it back and check for dropped frames or recording errors.

The first run is fast, then later runs slow down

Some cards have a fast temporary write area. Once that area fills, the speed can fall to the card's longer-term rate. A repeating decline can be normal for a card with a short burst of high speed, but it may make the card unsuitable for a long, high-bitrate recording. For more on checking write behavior over time, see how to test SD card write consistency.

Results differ a lot between runs

Background transfers, an active backup, or other programs using the card can change the result. A reader or USB connection can also limit the speed. Close other apps and try another port or reader before deciding the card is at fault.

The result is lower than the card's label suggests

Check what the label promises. For example, the Video Speed Class V30 indicates a minimum sequential write speed of 30 MB/s under the class's defined conditions. A browser measurement may differ because it uses a different setup, and the reader may be the limiting part. The card's class rating is not a promise that every device will record at that speed.

The card disconnects or the test reports errors

That is more concerning than a low but steady number. A loose adapter, dirty or damaged contact, failing reader, or damaged card can interrupt a transfer. Stop using the card for important recordings until you have checked it with another reader and copied off anything you can still access.

How to improve or isolate slow write speed

1. Close transfers and other demanding apps. Pause backups, downloads, and file copies before testing. These can compete for the same storage or connection and make results harder to compare.
2. Connect the reader directly. If it is plugged into a hub or dock, connect it to a computer port instead. Try another port and, if available, another reader that supports the card's speed class.
3. Check the recording's data rate. Video bitrates are often listed in megabits per second, while storage speed is commonly shown in megabytes per second. Divide the bitrate by 8 to convert: 100 Mbps is 12.5 MB/s before overhead. Leave extra headroom rather than matching the listed rate exactly.
4. Compare the card and reader with the device. A fast card cannot overcome a slow reader or a camera that supports a lower transfer mode. Check the device manual for supported card types and recording modes.
5. Try the card in its intended device. Make a short recording using the resolution and frame rate you normally need. A card can pass a brief computer test and still struggle with a sustained camera workload.
6. Format only after backing up. If the camera reports file-system problems, copy your files elsewhere and format the card in the camera or device that will use it. Formatting erases the card and does not repair failing memory or prove its capacity.

When the SD card may be faulty

Suspect the card if it repeatedly slows sharply, disappears, reports write errors, or produces damaged files in more than one reader or device. First copy off any files that remain readable. If the card is new and its actual capacity or write behavior does not match what was sold, keep the test details and contact the seller or manufacturer about a return or warranty claim. Do not rely on a card that has already lost recordings.

Speed is not the same as capacity or endurance

A write-speed result measures how quickly data was written during that test. It does not verify the card's full advertised capacity, predict how long the card will last, or guarantee that every camera will support the same speed. If your concern is whether a card holds the capacity printed on its label, use a separate capacity check and back up the card first.

The test runs in your browser, and the card's contents are not sent to a server.