I was moving my Atomic Pi around the other day, and I accidentally hit the BIOS/RTC reset button! Somehow, this made the eMMC flash storage unbootable; it simply didn't show up in the BIOS boot options. Further investigation showed that the boot flag was somehow deactivated! I made a recovery OS USB drive, and thankfully it's working again. But don't hit that button! I actually installed the OS to the eMMC and the MicroSD card, just in case.