I purchased a cheap two drive USB enclosure in order to setup an external drive that had RAID-1 so I could backup photos and recordings.

First, I formatted both drives. Then I ran extended smart self-tests to ensure I had decent drives. With RAID-1 and two drives I can only tolerate 1 drive failure.

Next ensure mdadm is installed: sudo apt-get install mdadm

Determine which dev devices the disks show up as. Next, create the raid device pointing at the correct dev directory.

sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdx
/dev/sdy sudo mkfs.ext /dev/md0

The device will start a resync process which on my system takes a really long time (days). If you want to avoid this initial re-sync you can use --assume-clean to avoid this. I would recommend letting it resync.

And there ya go.