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PPC hardware can only boot from APM (Apple Partition Map) partitioned drives, Intel-based macs can boot from APM, GUID, or MBR partitioned drives. But the MacOS X installer disc will only install onto GUID disks on Intel. However, you can take an installed volume from a GUID disk, and clone it onto a APM drive and it will be bootable.

With 10.5 this is not necessary, since you can install from the PPC computer onto the APM partitioned drive, and it should work fine on the Intel computer (assuming that that version of the OS supports that hardware). To be on the absolutely safe side I would manually download the 10.5.8 combo updater from http://apple.com/support/downloads, but that is being a little paranoid.
 
This means that i have my external formatted to APM and can clone both PPC and Intel drives. In the event of a failure the Intel can be booted from this drive and also cloned back.
Yes. That is how I do my backup clones.
 
"realised today that i don't have a backup of my macbook - 10.5.8
am also in process of reassembling my old iMac G5 ALS - 10.5.8 also
Appreciate that from a boot disk perspective then the intel is a guid format and ppc is on apm.
Is there anyway to have a single external disc to backup both."

An Intel-based mac can boot from a copy of OS X that resides on a drive that uses the Apple Partition Map.

HOWEVER -- you cannot INSTALL an Intel version of OS X directly onto a drive that is APM. You can ONLY install the Intel Mac OS to a drive that is initialized as GUID.

I believe the solution the OP is seeking is to do this:
1. Erase and initialize the backup drive, and format it to APM
2. Create 2 partitions as required to hold the backups
3. Use CarbonCopyCloner to create the backups on each partition.
4. Do "test boots" from each partition to see if the clones are successful, and that they can boot the respective computers.

Of course, the recommendations to use separate drives for each Mac will work, too.

Here's a suggestion to the OP for a 2-drive solution:
1. Get one of these:
http://www.amazon.com/Syba-Connecla...?ie=UTF8&s=electronics&qid=1253062702&sr=1-22
(note: many other versions of these docks are available -- the one shown is the one I use)
2. Get two "bare drives" of your choice.
3. Use the USB/SATA dock to create clones of each drive.
4. The USB/SATA dock will also boot the Intel Mac. Not sure if the older g5 iMac will be able to boot from it, however. It _will_ be able to read and write to it.
 
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