I have a Samsung Memory Card MicroSDXC 128GB PRO Plus UHS-I Grade U3 Class 10.

When testing it using CrystalDiskMark 5.5.0 x64 on Windows 10 using the Conceptronic Card Reader (product code CMULTITRVU3) via USB 3.0, I get the following Write speeds:

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This card is advertised as having transfer speeds of up to 95 MB/s for reading and up to 90 MB/s for writing.

Why am I getting such slow Write speeds and how can I fix this?

  • This seems about write. The maximum write speed is 90MB/s and the minimum (when an erase is required) is 13MB/s. So 24 is in the zone. – David Schwartz Nov 13 '17 at 2:03
  • @DavidSchwartz: Where did you find the 13MB/s speed when erase is required? 24 is in the zone, but much closer to 13 than 90. This still seems awfully slow for a U3 grade card. Is the other answer by Kang Jeonghoon wrong in saying the sequential write speeds should be at least 30 MB/s? – ProgrammerGirl Nov 13 '17 at 10:51
  • I got the 13MB/s speed from other people's benchmarks and they claim the erase speed is around that level. You are correct that U3 speed specifies a minimum sequential writing speed of 30MB/s. – David Schwartz Nov 13 '17 at 17:26

Is this your card: https://www.samsung.com/us/computing/memory-storage/memory-cards/micro-sd-pro-128gb-memory-card-w-adapter-mb-md128da-am/ ?

If so, sequential write speed supposed to be at least 30MB/s because it is U3 grade. See this link. So you should try some test if card is normal.

  1. If you use front USB 3.0 panel of your PC, try using back side USB 3.0 panel instead. Some PC has poor performance/power draw problem on front USB panels.
  2. Try using another SD card reader or test it on your tablet/phone. Your reader supports USB 3.0 speed so it should handle 100 MB/s with no problem.
  3. Try contact your seller. They should replace/refund it unless it is too late.
  4. If above doesn't work, you should RMA it to Samsung. They provide 10 year warranty.

Hope this helps.

  • Yes, that is my exact card. I tried your solution 1) above, but got the same results. I don't have another SD card reader to test and my phone does not have a card slot. Is there any other way to confirm whether the problem lies with the Conceptronic Card Reader (product code CMULTITRVU3) or the Samsung card itself in order to determine which I need to replace? – ProgrammerGirl Nov 13 '17 at 10:49
  • 1. You can see if it is operating with USB 3.x. Check this. Also check port is blue-colored port. If it is under USB 3.x category, then you can try an different card to check if card reader is normal.Or if it shows under USB 2.0 then check that port with another USB 3.0 sticks and see the info above. Might be a problem with a internal PC connection. (Port color is blue but actually it connected to USB 2.0 port of motherboard.) – Kang Jeonghoon Nov 14 '17 at 0:27
  • 2. If another card write speed fits in the class definitions link then it is the evidence that something wrong with samsung sd card. – Kang Jeonghoon Nov 14 '17 at 0:35
  • It's a proper USB 3 port. I don't have another high-speed card or reader to test, unfortunately. I have contacted the manufacturer of the Card Reader so see if their reader may be the limiting factor here. I will let you know what they say. – ProgrammerGirl Nov 16 '17 at 11:46

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