Just upgraded my internal drive size in my iMac 6.1 to 3TB. Had to partition it (partition 1) and did the swap out of data from the old disc to the new much larger one. Went to upgrade to Snow Leopard and (currently on 10.5.8) and both the external WD 2TB drive and the new internal 3TB drive have the " ! " showing on both drives. I go to DiscUtility and I can't even erase the new internal disc and start again. It wont show many options as it looks like it's sorta locked out from touching? The external WD drive is more open and it could be partitioned but this then requires a deletion of data on the drive...not up for that. I needed to have clicked on "GUID partition Table" when I first set up the new internal drive ??? Can you guys point me in a direction so I can get back on track and move forward with Lion after SN in all set? Could I add SL to the old MAc disc and then clone all the data onto the larger drive....hmmm don't think the old mac drive has enough space left. Stuart..