Drobo support 3GB drives now, although i'll say, i just got rid of mine. I had a drobo FS, and i think the processor and memory they put in there can't really handle the load of dealing with the raid and managing the file sharing at the same time. it never really got faster than an external USB drive.
I just replaced it with an external raid box from
sans digital. connects via USB3 or e-sata. i'm hooking it up to a linux box so i have e-sata, it did play well in testing to my macbook pro over USB2, but the speeds were limited by USB (30 MB/s).
this box will let you pick the raid level, anywhere form JBOD (5 separate disks) to RAID5 with a hot spare, so 3 drives of data, 1 active backup, and if one of those fails the last one jumps in and joins the group.
and this box was less than half of the drobo.
but... the drobo supports miss-matched drives, and is auto-expanding, so if your run out of room, just pop in an additional drive or replace a smaller with a bigger drive, and you're good to go. It's a lot more forgiving than the other raid setups out there.