The external HD does need to be partitioned with the "GUID Partition" Scheme. Otherwhise the Intel Mac's can not boot from it. Your external HD is partitioned with the "Apple Partition Map" Scheme, which will only work for the PPC Mac's to boot from. Both PPC and Intel Mac's can boot from the new "GUID Partition" Scheme.
Only solution:
- Copy the file's on the external HD to your iMac
- Repartitionate the HD with Disk Utility
Make sure you use the "GUID Partition Scheme" from the options menu
- Put back the file's on the HD
- Boot from the installation DVD
- Now the installer will be able to install OSX on it.
It is not possible to convert the "Apple Partition Map" Scheme to the new "GUID Partition" Scheme.
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