I've reinstalled Ubuntu, and somehow I messed up the old partition. I can see it in the disk utility. It's the 253GB one that's highlighted below. When I try to use one of the disk recovery tools (ddrescue or test_disk) they ask me for an approximate location of the lost partition. The problem is that I don't know how to get that approximate partition.
As you can see, this is considered a peripheral device, and that's only because the 2 500GB hdd are running in RAID 0. (This also means that gparted cannot recognize these partitions, it only sees the two 500GB disks.)
Anyone know how I can estimate where that partition starts and ends?