Time Machine Partitioning

My 1TB iMac completely filled up a few weeks ago. I deleted some files and got down to 830 GB, but needed an option for moving some stuff off the hard drive.

Got a recommendation for Drobo, so now I sit with a Drobo Mini with 2 x 1TB drives. I followed this tutorial to create a partition for Time Machine, designating TM as 1TB and the other Volume (which I plan to use to keep the files I’m trying to get off my iMac hard drive) at 15 TB.

So, TM is successfully on the partitioned Drobo drive BUT I have a red light on one of the empty bays, because that drive is nearly full. That drive will always be nearly full, because TM will save backups as far back as it has space for, correct? Can I just ignore that red light? Is there a way to turn it off?

Because the TM volume is almost full, copying files to the other partition is extremely SLOW! (it has moved 550MB in the last 2 hours and says it will take 36 days to copy the remaining 170 GB). This seems to be why copying is slow:
http://support.drobo.com/app/answers/detail/a_id/313/kw/slow/related/1 But it is extremely frustrating because my “Files” Partition is practically empty and the TM Partition will always be practically full.

Any tips on how to set this up more efficiently?