[Filmo]

I've put a Western Digital 320GB WD Caviar Blue drive into a OWC enclosure and am having problems getting it to act as an external boot drive.

I read that certain External WD drives are not suitable for boot disks, but was not sure if that applied to all WD discs, particular the ones for use internally as well. ??

I've got a intel iMac 10.5.8
OWC Mercury Elite Pro AL case with two disks in it, one of which is the 320GB WD Cavair Blue drive. Connected to the computer via Firewire 800 and also tried Firewire 400.
I've tried installing both 10.5.x and 10.6.x on the external.

When I 'boot' to the external, it just comes up with a gray screen. (Not the gray screen with the inlaid gray apple, the one before it that's just pure gray).

[Eriamjh]

Not sure, but does the disk need to be formatted as APM as opposed to fat32?

[seanc]

For an Intel Mac, the partition map would need to be GPT.

[P]

seanc is correct. APM, Apple Partition Map, is the old format for the partition map, not used for modern Macs. Instead, one uses GPT, GUID Partition Table. The partition table format commonly used by Windows is called MBR, Master Boot Record.

Fat32 is a filesystem, not a partition map format. It is actively discouraged by MS to use it for boot drives, but still works with certain limitations. The common filesystem on Windows bootdrives is NTFS, and has been for almost 10 years now. Macs of course use HFS+, also known as Mac OS Extended or Mac OS Journaled, for booting, although others (UFS, UDF, minor variations of HFS+) also work.

[CharlesS]

While the drive has to be formatted using GPT for the Apple installer not to refuse installing onto it, if you manage to get an OS X installation onto an APM-formatted disk somehow, it'll still boot on an Intel Mac just fine.

[Big Mac]

Why are we even talking about APM if we're dealing with an Intel iMac?

[Eriamjh]

I couldn't remember which one. The OP hasn't responded regarding format.

If you clone OSX onto an apm drive it might boot.