A while back I bought a 4-bay Drobo. FWIW, it's still sitting on my desk. Hasn't thrown a light or any errors. Slow as f*ck but rock solid.
How slow? How about 8-10MB/sec slow? Yeah. That slow.
But it houses my 1.5TB of files which include all my full-layer-intact Photoshop TIFF files (250GB) and 1.3TB of images. By the end of the year I should see this double in size. Accumulating full intact-TIFFs really taxes the system since the files average 450-500MB per retouched file. With my retouching speeds increasing, I'll need more space. Plus I've only been keeping full files since May 2010 so going forward my storage needs will grow faster.
I might seek to upgrade this primary storage (the Drobo) in the future. For that I'll considering a G-Drive 4-bay or OWC 4-bay Raid 5 or Raid 10. But if it ain't broke... plus I feel the lack of speed since it's really just archived stuff for the most part. My working files are on the scratch disk in the MBP.
What I'm looking to do now is provide myself peace of mind for the 1.5TB I have to protect. Currently I CCC (CarbonCopyCloner) to an external 1.5TB. I'll be purchasing 2 2TB Barracudas soon and rotating these disks in and out of an off-site location in case my house gets firebombed. I'll also be CCCing the boot drive on my MBP to the offsite location as well, even though I have a Time Machine drive sitting on my desk... again, just in case my house gets firebombed.
I will do this with a Voyager Q, 2 250GB 2.5" external, and 2 2TB 3.5" external drives. Rotating them in and out.
Once my files surpasses 2TB on the Drobo, I'll be purchasing 2 2-drive enclosures. Then I'll purchase 2 more 2TB drives to have 2 functional backup units that I'll rotate in and out to serve as backups.
This should allow me to double my storage and have 2 backups at all times.