I've tried 2 different classes and they both worked well. The one I'm using primarily is a class 10 32GB MicroSDHC and it works well. The other is an *old* 8GB SD Card I got with a cell phone and that booted without issue as well. I installed the Android image to the 8GB and Ubuntu to the 32GB The higher the class the faster the load times and write times, which is why I opted for the class 10 for the Ubuntu image
a class 4 or higher should work, but i would say the higher the best. Class 10 would be the prefered for best performance.
Don't waste you're time. The SD images don't match their hashsums even after several downloads. I've tried to boot this thing for two solid days now to no avail. The SD card took close to 6 hours to write? Clearly something is wrong. Followed the troubleshooting guide. Have a Fat32 card. Correct voltage and amps. Tried several monitors an TVs. Did notice that the website specifies a different minimum voltage than the warning on the back of the box. At this point I think I've been taken for $130. Good luck. I won't be back.
Does anyone know if there is a size limit on the card? I see someone using a 32gb which is comforting but if I'm going to get a class 10 card, I'm tempted to go for the 64
The block diagram shows the SD card connection as uSDHC3. This would imply Udoo only support SDHC card which has capacity of up to 32GB.