Hi Roger,
Here's the forum thread - note the response which says:
What you need to do when you get your second drive is simply insert into the system and then from Storage Manager, add it to the *existing* volume. As it is a SHR volume, it should start to put into usage, which means begin to mirror the first disk.
After the mirroring operation is finished, the disk type should become 'SHR with 1-disk protection'.
roger06 said:
Thanks sounds good. I guess it would be prudent to backup up anyway...
Absolutely - mirroring protects you against a hardware failure, but it's not a backup. As philipjohnwright says, it doesn't protect you from theft / damage to the unit, nor does it protect you from something simple like corruption or accidentally deleting a file (since the file is instantly corrupted / deleted from both disks).
Best practise would be to use RAID to protect against hardware failure, and then regular backups (ideally stored at another location, but at the very least in a different room of your house) to protect against other mishaps.