just wanted to add that i have the exact same issue as OP on 6.11.2, it appears to fail to format when starting to add to the array as a replacement drive. starting in maintenance mode, running a filesystem repair and then taking it out of the array and mounting in UD also fails. i upgraded from 6.10.3 last week, i may have to roll back to continue replacing the 2 drives with my 2 new disks.
EDIT: updating to 6.11.3 fixed the issue.