Your situation sounds frustrating and the Drobo Pro sounds slooooowwww!
The RAID 5 build times using an Areca ARC-1880ix-12 card 10x3TB was 7 hours + 35 minutes, or about 15 minutes/TB; quite a bit faster than 24 to 48 hrs/TB. Areca and their competitors also allow for the RAID to continued to be used during operation (reads/writes) although at a reduced performance speed.
I know it would be really painful to cut over to a controller card backup system with hot-swap drives or an external cable connected box (Sans Digital, etc.), but you may want to start dreaming / planning in that direction.
Sorry for your pain on this one.
24 to 48 Hours per TB is horrible even for the onboard Intel controller. A real SAS controllers is 6 to 8 hours for the TB storage Listed by JE. The onboard Intel controller takes about 72 to 90 Hours for that same storage. That is normally about as bad as rebuild times get. The rebuild time expected by Drobo is not even feasible as a storage solution. If that is the expected time then it's definitely time to start looking for Alternative storage.
Drobo finished rebuilding this morning and all seems fine, just nine days for the rebuild- could have been worse!
A good job that you didn't have another hard drive failure during the process!!!
I had the Drobo set to cope with two disc failures so I might have been ok. I'm not going to try it though