Two months ago, Apple announced Boot Camp, a preview of a utility to be included in Mac OS 10.5, Leopard. Boot Camp is a utility that allows users of Intel-based Macintoshes to install a Windows XP boot partition on their machines.

Sure, that's great. But say you're in the business of installing custom disk images onto lots of computers, some of them Intel Macs and some of them PowerPC. An Intel Mac won't boot from a PowerPC version of Mac OS X, and vice versa. So, you thought you'd install the Intel Mac OS X on your external FireWire drive. Right?

Wrong. You probably got a big "X" over the drive when you tried to install Mac OS X from the installation DVD. This "X" means that Mac OS X can't be installed on that drive. What gives?

As this website points out, there are going to be some problems if you don't format the external drive correctly:

The Intel-based Macs are the first Macs to use Intel's Extensible Firmware Interface (EFI). Originally intended to replace the PC's aging and unloved Basic Input Output System (BIOS), EFI has found its way into the new Macs, taking over for Open Firmware in PowerPC-based Macs.

Tagging along with EFI is a new partition scheme: GUID Partition Table (GPT); GUID itself is an acronym, expanding to Globally Unique Identifier. GUIDs are locally generated, world-unique random numbers, which make them handy for uniquely identifying all kinds of things without a centralized organization or database. GUIDs are a great way to identify hard disk partitions, enabling the operating system to track volumes even if the device interface changes (as would happen if you ripped your old hard drive out of your Mac and tossed it into a FireWire enclosure).

GPT replaces Apple Partition Map (APM) as the boot partition scheme for Intel-based Macs. And therein lies the rub. Intel-based Macs can't boot from older APM drives, and PowerPC-based Macs can't boot from newer GPT drives. This appears to be a permanent situation - each scheme makes incompatible assumptions about the layout of physical block 1 on the disk. While GPT was designed to be compatible with Master Block Record (MBR, the PC's old partition scheme), it doesn't play nicely with APM.

When you go into Disk Utility (/Applications/Utilities/Disk Utility) to erase the FireWire disk, unbeknownst to you, Disk Utility is formatting the drive using the APM scheme by default. To change this option, click on the "Partition" tab and then the "Options" button, where you will be greeted by three partitioning options (in addition to the ones already available in the "Erase" tab): Apple Partition Map (the default for PowerPC-based Macs), GUID Partition Scheme (the default for Intel-based Macs), and Master Boot Record (the default for Windows and DOS-based computers since time immemorial). If you want your FireWire drive to be a bootable volume for your Intel-based Mac, choose GUID Partition Scheme, and suddenly you'll be able to install Intel-based Mac OS X on your external drive.

Great! Woo! But what if you support a bunch of users who use both Intel and PowerPC-based Macs? Disk Utility won't let you partition a disk in multiple ways -- at least, not in one fell swoop. Here's a link to an article detailing how to install both Mac OS X for Intel and PowerPC on different partitions on the same physical disk. It requires several steps and takes a while, but it appears to work. Plus, it saves you from having to either (1) buy two FireWire drives and/or (2) cart two drives around with you.