I have two twinned RAIDs which are working just fine although the
second drive for both RAIDs is missing. After all, that's what it is
supposed to do -- work when things are broken..

The RAIDs are mdadm-style Linux software RAIDs. One contains a /boot
partition; the other an LVM partition that contains all the other
partitions in the system, including the root partition, /usr, /home,
and the like.

Both drives should contain the mdadm signature information, and the
same consistent file systems.

Each RAID is spread, in duplicate, across the same two hard drives.


EXCEPT, of course, that one of the drives is now missing. It was
physically disconnected by accident while the machine was off, and
owing to circumstances, has remained disconnected for a significant
amunt of time.

This means that the missing drive has everything needed to boot the
system, with valid mdadm signatures, and valid file systems, except,
of course, that its file system is obsolete.

If I were to manage to reconnect the absent drive, how would the
boot-time RAID assembly work? (/boot is on the RAID). Would it be
able to figure out which of the two drives is up-to-date, and
therefore which one to consider defective and not use?

Do I need to wipe the missing drive completely before I connect it?
(I have another machine to do this on, using a USB-to-SATA interface).

-- hendrik