Upload & Sell: On
At this point, I'm pretty confused as to what you're trying to do. It seems like you're trying to do Time Machine backups with disk images, which probably won't work, as those are incremental backups that take place every hour. I could easily be mistaken as to what you're doing though.
Here's what I would do: Get a separate one, two or three TB external drive and use that exclusively for your Time Machine. Then use a single volume on your Drobo with appropriate folders for your data files. Then, you'll need a backup for your Drobo, which could be another Drobo or a completely different brand. But you absolutely need to back up whatever is on the Drobo in case you have a catastrophic directory failure and lose all the data. That shit can and does happen. Much different than losing a single drive and popping in a new one. Of course, all that ain't cheap, but unfortunately, it's pretty necessary. Then, when you fill those up, you get to do it all over again. Fun!