[ale] Mirroring drives without RAID

[Alex Carver]

Got an unusual question from someone at work.  He wants to set up a pair
of drives to store test data which is usually of significant volume
(several tens of GBs per test).

He doesn't want to use RAID so that any single drive can be plugged into
any other computer to mount and recover the data directly.

The first thought was just a daily cron job rsyncing the two drives but
I wanted to ask if there was some kind of solution that faked a mirrored
RAID onto two drives where each drive is just a standard
partition/volume set (i.e. it's composed only of ext2/3/4 partitions and
not a RAID partition/volume)?