How to Check SD Card Speed
It is difficult to determine an exact speed for an SD card because transfer speed onto flash, or solid state, memory naturally varies. Cards are grouped according to "speed classes" that guarantee a minimum transfer rate so they can be reliably used for tasks like recording video. You can use drive-transfer-rate testing software to find a write speed for your SD cards since the software will recognize recognize the card as a drive. Run several tests and average the results to have a better idea of how your card generally performs.
Visit CNET's Download.com and search for a program called "Disk Bench." This program determines the speed of a tested drive by copying a small amount of information to it and carefully measuring the amount of time it takes. Click the green "Download Now" button beside Disk Bench.
Click "Save File" in the window that appears and wait for the download to finish. Open your computer's download folder. Depending on the operating system you are using, you may have to extract the files from the zip folder. If so, right-click on the folder, then click "Extract Here." Double-click the "Disk Bench" icon that looks like a small hard drive.
Once the program opens, click the "Copy File" tab at the top of the window if it is not already highlighted. Click the ellipsis button to the right of the "Source File" line and use the window that appears to navigate to any small file and double-click it. Text documents and photograph files work well. Click the ellipsis button next to the "Destination File" line, navigate to the SD card you want to test and double-click.
Leave the "Kill Destination File" box checked unless you want a copy of the source file added to the card. Click "Start Bench" and wait for the program to run. Test time will depend on the size of the file you selected for "Source File." Disk Bench will display the test results in both time and megabyte transfer rate in the white box in the lower half of the window. Since transfer rates constantly vary with flash memory, run the test several times, ideally with different sized files, to find an average result.