Mac OS X 10.6: If you can’t install Mac OS X on an external disk
If the Installer won’t let you install Mac OS X on an external disk, the disk may not have the partition scheme required by the Installer. The installer installs Mac OS X for Intel-based Macintosh computers only on drives with the GUID Partition Table scheme.
You can change the partition scheme for the disk by repartitioning it. It’s OK to create only one partition.
To check an external disk’s partition scheme:
Open Disk Utility (located in the Utilities folder in the Application Folder), and then select the disk from the list on the left.
Choose File > Get Info, and look for the Partition Map Scheme.
If you don’t see the Partition Map Scheme, you may have selected the volume instead of the disk. (Disks are flush left, with their volumes indented below them.)
To repartition an external disk so you can install Mac OS X:
Back up any important data on the disk. Repartitioning it erases all its data.
Open Disk Utility (located in the Utilities folder in the Applications folder).
Choose the disk you want to partition from the list on the left, and then click the Partition tab.
If you don’t see the Partition tab, you may have selected the volume instead of the disk. (Disks are flush left, with their volumes indented under them.)
From the Volume Scheme pop-up menu, select the number of partitions you want, and then click Options.
Select GUID Partition Table, and then click OK.
Set any other options, such as names and formats for the partitions, and then click Apply.
After the drive has been partitioned, you can install Mac OS X on it, and then restore any data you backed up.