Hi. I had RHEL6.8 installed on a fake RAID1 on a machine with Intel Matrix Raid controller.
The array was built out of two 300Gb hard drives and managed by dmraid
At one point I needed to boot the machine from another non-empty 500Gb HDD. I replaced the drive in the first slot and turned the power on. Guess what, it kicked me into the recovery shell, because it could not mount the "/boot" entry from fstab, because the logical device sda1 disappeared during boot! sda2 was there and mounted just fine.
I removed the drive from the second slot, but it was already too late: the RAID metadata was copied to my 500Gb disk and during boot dmraid complained that it could not activate ddf...
It was dmraid who kept deleting sda1 during boot.
Later I found the solution
here, but now I just want to know who spoiled the new hdd, BIOS or the OS. One of them was too smart and automatically treated it as a spare, although it was non-empty and had different size