Using another HD is a mistake... tape is really the best way to go:
1) Media is CHEAP.
2) Backups can be automated, and
3) You can carry a complete backup away with you.
Using another hard drive is cheap, true- but automation will be tricky unless you don't plan to keep more than the most recent backup- which will be troublesome, as you'd be surprised how often the most recent backup tends to include exactly the bad data you're hoping to replace, making it necessary to go back to a previous backup catalog to retrieve the "good" version. Reliability of HDs versus tape media is also quite low. And, perhaps most importantly, if you're keeping your only backup not only on-site but actually inside the machine you're backing up, you're putting the whole shebang at risk of being destroyed by machine failure, not to mention the risk of natural disaster, fire, etc. at your location. Best practices for backup, therefore, should include off-site storage... just carry a set home with you each time you back up, so that when you finally suffer that fire or flood or whatever, you can restore data just as soon as you replace the server.
LevcoS