Testing your SD card writing speed is important if you have multiple SD cards and only want to use the fastest ones. While there is no official good or bad writing speeds for SD cards, there will be a difference in speeds, even between cards of the same size from the same manufacturer. If you want to be sure you are always using your best cards first, testing is the way to go. The simplest test would be to just write a large file to each card and see how long it takes; however, if you are looking for more thorough tests, there are a few software programs that can help.
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The Crystal Disk Mark software is a bare-bones speed testing software. You simply select the SD card's drive letter in the upper right corner of the program and then start the program by pressing one of the green buttons on the left side of the program. Crystal Disk Mark will measure the sequential writing speed, as well as random writing speeds.
For most purposes, the sequential writing speed is the number you are looking for, so you can save some time by just running that test. The random tests are only important if you plan on writing a lot of tiny files. Regardless of your SD card, the write times will be slow, due to how flash memory works. Once Crystal Disk Mark has finished the test, you will see a green bar with a number under the "Write" column. This is the writing speed, measured in megabytes per second.
HD Tune Pro was designed to determine hard drive speeds, but you can use it for SD cards as well. To get access to writing tests, you need the "Pro" version of the software, not just "HD Tune." You have to pay for the Pro version, but the software comes with a 15-day trial period.
After you open the software, select your SD card from the drop-down menu at the top of the program. Then click on the "File Benchmark" tab. Press the "Start" button to begin the test. The test results are in graph form only, but you can copy the numbers to a clipboard with a button at the top of the program. HD Tune Pro runs the test with 15 different block sizes, so ignore the first six results on the graph, since these will be unnaturally low for almost all flash memory. The rest of the results should all be close to each other and will represent the approximate writing speed.
Flash Memory Toolkit is designed as a comprehensive flash memory suite, complete with ways to find errors on memory cards, recover lost files and test card speeds. To test an SD card, select "File Benchmark" from the list of programs on the left side of the window and choose your SD card from the drop-down menu at the top. Click the start button and the testing will begin.
Flash Memory Toolkit will test five different files sizes, all large enough to be considered good tests for flash memory. The writing speed will be listed on a yellow bar on the graph output. The average of all five tests should give you a good idea on the overall writing speed of the memory card.
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