Now on my second brand new unit I updated to 7.5.2 first, then placed the 3TB drive in. The referb is what I refered to originally in this thread, where I installed the new drive then updated the firmware (from 7.5.1 that it shipped to me with, to 7.5.2). If you don't update beforehand, you'll probably need another time capsule to write the proper partition tables to the drive. first setup and update your new A1355 time capsule to 7.5.2 BEFORE ripping it apart and putting in a 3tb drive. My A1254, however, was already updated to 7.5.2 and everything worked. I also did not update the A1355 to the new 7.5.2 firmware before slotting in the 3tb drive. Perhaps someone else can chime in with more information. I know for local disks, they must also be formatted as HFS+, but I don't think that applies to network volumes (since, for example, lots of NAS units support Time Machine over AFP, and none of them are formatted as HFS+ volumes). I don't know if Windows Server's AFP support is adequate.

SMB or other protocols, even if the Mac can connect to them just fine, won't work. The variables I know of are: Time Machine will only back up to volumes shared via AFP (Apple File Protocol). Sorry I missed your reply This is a good question, and I'm not 100 percent certain of the answer. Would this be possible on a drive running different OS (win 7) ? What network sharing protocol for that drive I would need to enable so that the laptops discover that volume as available for backup? Right now, I have a 1.5TB external USB2 drive connected to my iMac with two partitions: 1.2TB and ~300GB. I've had great success (over a year using multiple hard drives and configurations) backing up my girlfriend's MacBook, wirelessly, to a volume shared off my iMac.

Airport utility 5.5.2