In Pursuit of Micro SD-Card Speeds – Part 2

A few weeks ago I wrote about a magazine article I found in which micro-SD card speeds were evaluated. In the article, amazing speeds were mentioned that I could not reach with my USB2 Micro-SD adapters. So I went out and bought a small USB3 reader. I turned out that while read and write speeds improved, they were nowhere near those in the article, despite using the same Micro-SD cards. So I bought a ‘Ugreen USB 3.0 SD-card reader‘ with a built-in USB-3 hub that got good reviews for its read and write performance.

And indeed things significantly improved and interestingly varied significantly and unpredictably with the SD cards I had. The numbers below are all for the same Sandisk Ultra branded micro-SD cards with a size of 16, 32 and 64 GB, bought at different times over the last year.

  • 16 GB micro-SD card: Read: 80 MB/s, Write: 10 MB/s
  • 32 GB micro-SD card: Read: 80 MB/s, Write: 40 MB/s
  • 64 GB micro-SD card: Read: 80 MB/s, Write: 15 MB/s

What is interesting and disappointing at the same time is that the write speed of the 32 GB micro-SD card was significantly faster than that of the 64 GB card which I had only bought very recently. I was assuming that write speeds would scale with capacity. Well, obviously it doesn’t, perhaps Sandisk doesn’t always use the same flash memory for their ‘Ultra’ branded micro-SD cards.

And here are the two commands I used to check the write speed. One of them writes zeros to a file while the other erases the micro-SD card by overwriting it with random data:

sudo dd if=/dev/zero of=/media/martin/64GB/test.txt bs=4k conv=notrunc

sudo shred -v -n 1 /dev/sdb