I have the same.
First of all, kudos to you for knowing that fault-tolerance (which Drobo provides) is not the same as backup!
In my case, I have all my DVD discs readily accesible and I have an autoloader, so I wouldn’t cry too much if I lost my on-disk archive. I could always re-image the collection.
But, if your discs are locked away or otherwise not easily accessible, it definitely doesn’t hurt to have backup (except for the hurt on the wallet).
You’d want to make a backup of either:
a) The data
b) The volume
Backing up the data is the easiest. Unless you’re using some weird proprietary method to transfer the data from your DVDs to the Drobo, you should either have disc-image files (.ISO files) or folder structures (VIDEO_TS and AUDIO_TS folders for each disc). Both are easy to copy to another storage medium, be it a single large volume like another Drobo, or multiple smaller volumes like portable USB drives or “loose” hard drives.
Backing up the volume is a little trickier. Mainly because it’ll generate an immensely-large image file, as the MPEG-2 compression on DVDs won’t compress very much more than it already is. So really, you’d need a volume large enough to handle your library.
Unless you’ve recompressed your library to some other, more-highly-compressed format, backing up to DVD-R would be foolish, as it’d be easier to just re-image the original discs.
If I was to back up my media Drobo, I’d get another Drobo and use that as the backup mirror.
Hope that helps!