It sounds like one lucky soul has replaced all 4 of his drives with 2 Tb drives, and was able to create a RAID5 array, though in his post he did not explicitly state that he got the anticipated 6 Tb of space (3 drives worth of data plus 1 drive worth of parity to allow the array to recover if one drive fails).
OTOH, I' ve been running with four WD Caviar Black WD1002FAEX drives for 18 months without incident, until one of the drives appeared to fail. I asked WD to replace it under warranty, and they did not have any re-certified WD1002FAEX drives to replace it with, so they sent me a WD2002FAEX (2 Tb) drive instead. Unfortunately, I have not been able to make the TeraStation DHTGL work with three 1 Tb drives and one 2 Tb drive. I've tried everything I can think of, including deleting the RAID5 array, formatting the 4 drives, temporarily connecting the 2 Tb drive directly to my computer's eSATA port and filling half of the space with a 1 Tb NTFS partition so the drive would have about the same amount of free space as the 1 Tb drives, re-creating the array, etc. (Yes, all the data are backed up elsewhere, so there is no problem with losing data, unless my backup fails while I keep fussing with this backup device.) I have no problem creating a RAID1 array with two of the 1 Tb drives, but cannot create a second RAID1 array with the 1 TB and 2 Tb drive pair. Of course, my original intention was to create a RAID5 array with all four drives, allowing half of the capacity of the replaced drive to go unused, but no such luck with that (which was actually the first thing I tried to do after replacing the drive).
Under Disk Mangement menu, all four drives are identified correctly (correct type, correct size), and I can format and/or test each of the 4 drives successfully as single drives, but as soon as I try to create a RAID array with two different-sized drives, it fails with an error message:
RAID Array error. The File System or Disk Format is not readable.
Looks like I'm going to have to buy a new 1 Tb drive after all, and put the 2 Tb drive into an external case for extra storage...