I found out to my disgust that Time Machine on the Mac only backs up your main hard drive, not even a second internal drive according to the apple tech support person. It definitely doesn't back up external drives (upon sit most of my important images). Ouch! That makes it near worthless to me.
I checked with the Drobo people - they recommended Super Duper - as did my friend Bill. Only catch is, Super Duper can update changed files but does not do incremental backup - so if the file is corrupted and then backed up - too bad - if you want an older version before you stupidly shrunk the original file for the net (I have done this), then so sorry, it's gone - start all over with the raw file. As I spend hours editing images and sometimes haven't a clue what all I did in which sequence to an image, this is mighty frustrating.
I may be forced to purchase and use Retrospect - which does in fact handle multiple drives and incremental backups. The standard version isn't all that expensive at $129.