GUID or Apple Partition Map

As long as you do not plan to use it to install a fresh version of OS X then it doesn't make a real difference. Intel Macs can boot from both APM and GUID partitioned drives. You just cannot install an Intel version of OS X on an APM partitioned drive.

GUID provides for on the fly altering of existing partitions non-destructively. It also permits the creation of both HFS and MSDOS partitions on the same drive.

1. My understanding is that Time Machine will not recognize a drive that is not GUID. Period.

2. Extrapolating from http://en.wikipedia.org/wiki/Applepartitionmap and some experiences of other users who were having trouble with Boot Camp because their drives were not formatted GUID, it sounds like if you wanted your external disk to be bootable for both your Intel iMac and your PPC iBook and mini, then you would want the Apple Partition Map. I won't claim that my extrapolation is accurate, but it's derived from the idea that an intel mac can boot from an APM partition, but I don't know if the PPC will boot from GUID. So because you don't intend to use the drive as a startup disk, you can use GUID anyway. 🙂

2.5 See http://en.wikipedia.org/wiki/GUIDPartitionTable GUID is for Intel machines and is an improvement over MBR (Master Boot Record).

Well, I would recommend go GUID if you are using an Intel Mac, and using the drives for Time Machine backup.

But *Mr. Lobotomy*, here's an interesting thing you might want to know - when I first connect my Maxtor One Touch 1TB FW 800 drive to my iMac for Time Machine backup, I formatted it as a normal Journaled disk (that was before I read about the GUID partitioning), the back up actually went well. i.e. Time Machine "sees" the drive and performed the back up as usual.

But after I read about GUID, I just reformat/repartition it and re-run Time Machine again, so far so good.

Cheers

And the answer is use GUID for Intel macs and use APM for PPC computers.
Use Mac OS X extended journalled.
The other thing you should know is about the proper computer name.
Mac OS X 10.5: Time Machine backups are not visible
http://docs.info.apple.com/article.html?artnum=306928

"Use "Apple Partition Map" partition scheme if the disk will be used with Time Machine and a PowerPC-based Mac.
Use "GUID" partition scheme if the disk will be used with Time Machine and a Intel-based Mac."

quoted from the Apple support site.
http://docs.info.apple.com/article.html?artnum=306932

APM does not work for both so you need to partition if the drive is big enough or get another drive.