The drive will most likely be partitioned from the factory MBR which is a PC native format. If you are going to be using the drive exclusively with an Intel Mac, it is best to repartition the drive GUID. If you need to use it with PPC & Intel Macs, partition the drive APM.
This would let you actually boot from the external drive later if you install an operating system on it. You never know when this may come in handy (like extracting data from a hard drive that will not boot).
Repartitioning is not done by default when Erasing a drive. You need to specifically partition the drive.
1) Open Disk Utility.
2) Select the external drive in the left panel. (Ensure you have the right drive!)
3) Click the Partition tab.
4) Change the Volume Scheme to the desired number of partitions. This will typically be 1 Partition.
5) Enter a name for the new volume.
6) Select the Format; typically Mac OS Extended (Journaled).
7) (Here's the key) Click the Options button below the volume layout graph and select GUID. Then click OK.
8) Click the Apply button. Remember that this will erase all of the data on the selected drive.
Start using your newly partitioned and formatted drive in good health.
Jeff
