I would stay far far away from the Drobo, what a DOG! Slow as a three legged dog.
Ok, on a more serious note, what are you wanting to do? obvious store your data, and given you are wanting RAID 6 I am assuming you are in the back of your mind thinking of this as a safer backup solution :-) while still being online and available.
Knowing that you are a Mac user, we have here an Xserve, with 14 disks and it is reasonable fast, but we should have used Optical Fiber links from the server to the work stations instead of Cat6 cables, but as a long term storage solution it isn't bad. Though it is on the expensive side.
We have reached the limit of the amount of storage we can fill this beast with, and bought a DROBO as a secondary storage solution with the network unit, but it is slow, so slow....If I have something online on the network, I don't want to wait till lunch time to access something, it has to be available within less then a minute tops.
So we have the Xserver, a Drobo and we will soon be running out of space. Some of the data we have is getting old and we are getting less and less request for it, so I am looking at keeping this data on hard drives that are off line, but with the use of an eSATA cradle, I think Jack Fletcher is using this solution, but i could be wrong, if so, my apologies! This idea will work with a good DAM setup, where the DAM application will remember on which disk the data is located. So the idea is to label the disks clearly :-) and insert the one into the cradle when you need. So what if the hard drive breaks down!? well, It would probably be a good idea to have this data on two disks one in the studio and one off site.
This way, I can keep what is current on my faster storage solution, and with my DAM application go and collect the disk with the old clients photos on and access them at the same speed as a normal hard drive.
What is needed,
1) Hard drives in pairs
2) eSATA card in your Macpro and/or Macbook Pro
3) eSATA cradle (usually also have USB, but slower), its small so you can take if with you on location, it supports 2.5" and 3.5" disks SATA and SSD, but limited to the eSATA port
4) DAM application of your choice
think about it, you already have about 10TB of storage, that is online and available immediately...how much of this is current data? I know giga-pano's will consume lots of space, but still...
just my $.02
PS: I also have an HP U2 server with a MSA (Mass storage Array) using SCSI U320, 14 disks but as we both know it may be good and solid, but its old technology and considering the cost 300GB SCSI disks I think i'll stick with SATA, but it does support RAID6
its forsale :-)