I recently needed to rebuild a RAID1 array after a reboot for some odd reason and afterwards I was unable to assemble the array. mdadm came back and reported “Device or resource busy” on one of the drives. I couldn’t figure out what the issue was originally as it wasn’t mounted and no other processes were using the drive via lsof. Eventually I tracked it down to a changed UUID – my fstab was trying to mount the old mdadm array and it locked the resource. I checked it by doing ‘ls’ on /dev/disk/by-uuid/. Updating fstab with a new UUID, rebooting to clean up things and reassembling the array solved it. Just a useful item to keep in mind.