i must say that
Seagate - Barracuda ST2000DM001 - 2TB (Standard Configuration) [Added 12/06/2012]
will format only if it is put ALONE in the NAS.
so if you have a previously fit hard disk in order to format the new 2TB unit you should remove the old HD, insert and format the 2TB disk, then replug the old disk.
thanks to user "daydream" for pointing this out!
this is quite duff !
[my NAS has FW ver. 2.02]