I ran into some problems trying to install Mac OS X on an external hard drive, to make a test clean install for booting up my Mac Pro. I was trying to use a drive that I’d previously formatted on a PowerPC Mac, and the Mac OS X installer disks that came with the Mac Pro wouldn’t let me install OS X onto the disk.

Turns out that the Partition Map of the disk has to be set to be using a “GUID Partition Table” rather than an “Apple Partition Map” in order for the disk to be used to boot up an Intel Mac. Not exactly obvious, but thanks to some previous posts on the Apple Discussion Boards, I worked it out.

To set up the old drive to boot up an Intel Mac, open Disk Utility, and select the drive in the left-hand pane. You’ll need to select the drive, not any partitions already on it. Select the “Partition” tab in the right-hand pane, and click on the “Options…” button at the bottom of the right-hand side pane. A new window will slide out, allowing you to choose a partition scheme. If this is set to Apple Partition Map, then change it to be GUID Partition Table instead. Click OK.

You’ll then need to re-partition the drive for this new setting to be used. This will erase the drive! Click on “Partition”, and the drive will be re-partitioned using the same partition names and sizes as before, but with the new “GUID Partition Table” partition map.

You should now be able to insert your Mac OS X install CD, restart the Mac from the install disk prompt, and install to the newly-partitioned drive.