Another thing that is easy to forget is that you need to make sure the hard drive has the OS 9 drivers installed. If you try to format a hard drive on a OSX only mac such as the FW800 MDD, the option to install the OS 9 driver has been removed. Use a drive pulled from a OS9 bootable G4.

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You are right. I totally forgot about this. I don't think either drive I tried has the OS 9 drivers.

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This is interesting - I just connected a new FW800 hard drive to my G5, and Disk Utility offers the option to install Mac OS 9 drivers when formatting.

The drive is an Iomega eGo 320 GB, it's a laptop sized drive that works off FireWire power (or 2 x USB port power) with FW400, FW800, and USB ports. It comes already formatted as HFS+ with the OS 9 drivers. I personally recommend this drive to anyone who's looking for one, it's $79 at Fry's right now.

I'm going to reconnect the drive using FW400 (since OS 9 didn't have FW800) and try booting off of it.