On the wiki page for creating a mirrored root drive, at <https://wiki.centos.org/HowTos/Install_On_Partitionable_RAID1>, it first tells you, in the install, to chose manual partitioning, and to leave at least 1M at the end of the drive.... and then it tells you to create the RAID 1 using /dev/sda and /dev/sdb - the *whole* raw drives. mark, currently creating a RAID 1 using just the raw drives, no partitions