In my USB Mirror solution post I had left out a simple, but vital piece of information.

When rebooted the system failed to find the RAID devices because it wasn't told where to find them. To get around this you need to use the mdadm tool to write out or amend the /etc/mdadm.conf file. The details are now in the previous post, but for completeness here they are again:

mdadm --examine --verbose --scan --config=/proc/partitions >> /etc/mdadm.conf

If you had a previous mdadm.conf then you will need to edit to remove duplicate entries, but otherwise you only need a little tidy-up. You will probably get an entry something like:

ARRAY /dev/md0 level=raid1 num-devices=2 UUID=c44c4585:f8b3e0ba:bc171fb8:9618c603
devices=/dev/sdc,/dev/sdb


You can remove everything that has an = in it (in other words the options) as these will be discovered by the raid subsystem provided it can find the arrays. So the final (tidied) version would be:

ARRAY /dev/md0 UUID=c44c4585:f8b3e0ba:bc171fb8:9618c603

So now my system is booting happily and running a full-disk software RAID solution. Next step is to prove this will work the same using USB drives on my main backup server.