Here’s something I learned the hard way.
Turns out Mac OS X’s Time Machine can backup external drives!
Why did I believe the contrary of this? I’m not sure…
Time Machine by default does not backup external drives, but it can easily be removed from the “Do not back up” list.

I’ve been using a Drobo as my live storage drive, and a Firewire disk for Time Machine backups.
Using a single drive based system for backups is not so great, but with Drobo’s seemingly unlimited storage (provided I keep feeding it hard drives), I wanted to use it for my main storage with the idea of buying a 2nd Drobo to replace the Firewire drive eventually.
While reimporting pictures into iPhoto, I noticed that some pictures have a watermark on them, which I do when I upload to websites for sharing as I’ve had my pics stolen before. But I always leave the originals in iPhoto untouched.
Somehow when I initially did the watermarking and uploading – I must have trashed the wrong folder. And because I was dumping all my pictures into Drobo for the initial processing is when this mishap happened. I do however keep a copy of (some) of my unwatermarked originals on my website as it’s how I do watermarking now, and I had assumed the originals I’m looking for were there. My assumption was wrong.
I didn’t lose the pictures, but they are permanently watermarked and downsized now.
So now I turn to my external drives and Time Machine to help me with this problem in the future.
Another item I didn’t know, is that Time Machine will happily use any HFS+ drive WITHOUT formatting them, and allow you to keep other data on it. So I immediately changed my “Time Machine Disk” to Drobo as I move things around (and no, Time Machine will not make backups of the Time Machine drive to itself).
Items I don’t care about or can easily replace, such as temp download files, ISOs to software I have on disk, etc will stay on Drobo (altho RAID – no archival backups).
Items such as pictures, documents and such that I DO want being backed up are being moved to the Firewire drive. Yes, a single drive for initial storage still makes me nervous, but should it fail everything will be backed up on the Time Machine drive as well. I will replace the Firewire drive with another Drobo eventually.