After finding we're relying on my (Linux based) media server for more and more (have nearly all our CD collection on it, plus downloads, videos, and photos), it seemed a good idea to have some backup.
Currently we've got about 300Gb on a 1TB USB drive used up.
So I bought another (identical, although it wasn't a requirement) 1TB USB drive, plugged it in, and after reading up on the mdadm software RAID management tool, I managed:
1) to create a RAID1 array, with the new disk, telling mdadm that the 2nd disk was "missing" 2) Copy data from existing disk to RAID array 3) Wipe partitions on old 1TB disk 4) Add old 1TB disk to RAID arrayThere was a slight warm feeling of satisfaction, when I saw the mdadm monitoring pick up the addition of the disk, and start rebuilding the array. And a quick test shows that even while rebuilding the array, the media server can happily pump out an HDTV file with no problem for the media player.
The only wrinkle is that it's not hot-swappable RAID AFAICS. If a disk goes belly up, I'd have to manually partition and add the new disk to keep things going. Still, not a bad result ...