This is a known issue in Pro Tools HD & LE 7.4.2 & 8 (Mac OS X PPC).
Workaround: Format the drive with Apple Partition Map partition scheme.
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.
It is in the Pro Tools documentation but it has caught me out!