From time to time I get the problem that the kernel cannot access the sd card when trying to mount the root file system. It happens mostly (but not always) when the Hummingboard is powered on for the first time after having been off for a few hours. A second attempt to boot usually works. I have not seen this on the Cubox-i, only on the Hummingboard with recent kernels. SD card is Sandisk Ultra 8GB.