The Drobo work by creating RAID sets across the drives in a mix of RAID 5 or RAID 1, depending on the size and number of disks. For example (stealing from wikipedia), a Drobo filled with a 500, 400, 200 and 100 gb disks would create a 100gb raid 5 across all 4 disks, then a 3 disk raid 5 across the 500, 400 and 200. At this point, the 200 and 100 disks are full, so it switches to making RAID 1 raids on the 500 and 400. With this particular setup, 100GB on the 500GB disk would simply be lost.

To calculate the usable space on a Drobo array, you would add up all the disk capacity (1200gb in this example), then subtract the largest size drive (500gb), leaving the usable capacity (700gb).

It's basically the same trick the ReadyNAS can do with XRAID2. In either case, it's always best to have 2 of the largest capacity drives in the RAID to ensure no wasted space.

*edit* Oh, one thing that bothers me with the way the Drobo works is that it asks you during setup to set the maximum size of the RAID, and allows this number to be higher then what is possible with the disks in the array. This can lead to some odd issues where a computer sees an 8TB drive, but can only write to 3.5TB or so of it. It's a way to work around file system expanding limits I suppose, but it still seems strange.