Folk have been having some problems when buying Mac friendly pre-formatted drives like a Seagate FreeAgent Desk for Mac as it comes pre-formatted with GUID partition map. I you have an Intel Mac then that will NOT cause you a problem. However if you have a PPC or you are going to end up at a facility that still has Power PC Macs you will need to reformat it to the Apple Partition Map before it will play with Pro Tools nicely.
This little problem slipped in with the introduction of Intel Macs and Mac OS 10.4.6 when GUID Partition Table support was introduced. The default mode when formatting drives now on Intel Macs is GUID Partition Table. If you put this drive onto a Power PC Mac like a G4 or G5 then you are likely to get 9131 errors in Pro Tools. You can check if your drive is partitioned with a GUID Partition Table using Disk Utility.
If you select the higher level icon in the drive list in Disk Utility, in the bottom section it will show what partition map has been used.
Any drive formatted on an Intel Mac will use a GUID Partition Table. You will need to use the Options menu in the Partition tab to force Disk Utility to select Apple Partition Map.
Alternatively just format the drive on a Power PC Mac and it will be formatted with an Apple Partition Map.
If you have an incorrectly formatted drive you will need to move all the material off it to reformat it with the correct partition map. So if you do buy a pre-formatted Mac friendly drive then check it with Disk Utility first and pre-format it BEFORE you put anything on it.
It is in the Pro Tools documentation but it has caught me out!
