I’ve been able to confirm that it is in fact possible to boot an Intel Mac and a PowerPC Mac from the same external hard drive, something that had been previously held to not be possible. The boot disks for the Intel and PowerPC versions of OS X do need to live on separate partitions, but the Intel partition can be formatted as APM, or Apple Partition Map, instead of the Intel-specific GPT, or GUID Partition Table. This is building on the great work of Jonathan Rentzch at http://rentzsch.com/tidbits/intelbasedMacBootIncompatibility.
What I did:
1. Partitioned a large Firewire/USB 2.0 external hard drive on my iMac with two partitions. Both were formatted as GPT, and were identically sized. One was named Intel Boot and the other PowerPC Boot.
2. Hooked the hard drive up to my test Intel iMac via USB and then booted the Mac off of the 10.4.4 install DVD that came with the iMac.
3. Installed 10.4.4 on the Intel Boot drive. (I left off the bundled software, so it was just 10.4.4.)
4. After the installation, I booted off of the external drive via USB 2 and made sure everything was working as expected. At that point, I also ran Software Update to make sure that I was up to date. Software Update found and updated my external drive with 10.4.5. Once the updates were finished, I booted back off the iMac’s internal hard drive.
5. Once booted from the internal hard drive, I used Disk Utility to make a cloneable image of the Intel Boot drive and saved that onto PowerPC Boot.
6. I disconnected the external drive from the Intel iMac, then connected it back to my G5 via Firewire.
7. I copied the Intel clone image from PowerPC Boot onto my G5. I then re-partitioned the drive again on the G5 and re-setup the Intel Boot and the PowerPC Boot partitions as APM-formatted partitions.
8. I then cloned the Intel clone image onto the Intel Boot partition, and cloned an existing 10.4.3 PowerPC image onto PowerPC Boot.
9. I booted my G5 off of PowerPC Boot and my Intel iMac off of Intel Boot. There appeared to be no problems.
This method obviously relies on building the image on an Intel Mac that has a GPT-formatted partition of sufficient size, but it does at least solve the problem of needing multiple external drives to support both Intel and PowerPC Macs. I used USB 2.0 in my testing mostly because I was testing the iMac’s ability to boot off of USB 2 as well. As far as I am aware, there’s no problem with booting an Intel Mac from FireWire.
Anybody had similar success? Let me know in the comments.