I have a similar setup, Mac mini connected to OWC 2TB external HD, with another OWC 2TB HD for backup. I store the backup at my office and bring it home roughly once a month, or whenever I add a bunch of music files to the main drive. I also use Carbon Copy Cloner for the backup.
A couple of weeks ago I brought the backup drive home and couldn't get it to spin. It's about 2.5 years old and OWC has a three year warranty, so I called and they told me to send it in. They ended up replacing the drive. They offered data recovery options (for a price, of course) but I declined since it was my backup. I was nervous about anything happening to my main drive until I got the second drive back and redid the backup, but it all worked out. I can see the logic behind multiple backups though. This was my first experience with HD failure, and it wasn't fun.