OK following on from this thread, I decided to perform a "transplant" operation. I need to take the 20" iMac in for repair, but before I do so, the hard disk needs to be backed up. As it won't power on, I thought I could swap the hard disk over to my 17" iMac and connect up my new LaCie FireWire drive to take a clean backup. I've taken some photos of this process, for all you geeks out there longing for a bit of iMac Pr0n (click on the thumbnails for a larger image) Patient X (left): a sick 20" iMac G5 Revision B Symptoms: will not power on. Diagnosis: suspected logic board failure. Prognosis: good; still under warranty. Complications: hard disk has not been backed up in a very long time. Patient Y (right): a healthy 17" iMac G5 Revision B who has kindly offered to act as a surrogate for the hard disk backup process. OK they're prepped and ready for surgery. Let's open them up: The hard disks are located to the top right of each machine. The 20" iMac's hard disk is removed simply by taking out three brass screws (as indicated below). The drive can then be lifted an inch or so off the midplane and the logic board connector, hard drive power and data cables can be disconnected, as shown. The 17" iMac's hard disk removal is a tiny bit more complicated since you have to first remove to screws from the fan cover to get to one of the hard disk screws, but after that the process is the same. Ok here are the two disks side by side: As you can see the enclosure of the 20" one (pictured left) is a fraction larger and the logic board connector is located to the bottom, whereas the connector on the 17" disk is on the right hand side. For this reason, the disk from the 20" iMac doesn't quite fit into the 17" The drive is held in by the cables attached to it, but because it wasn't quite fully seated and I was being rather cautious, I decided to ask the lovely people here at MacRumors if it were definitely safe to use it like this! Blue Velvet assured me it'd be fine and suggested trying to secure it a little with something before leaning it up to power it on. So here it is ready to go... *so* nervous about pressing the power button... w00t! It works a dream. All ready to back up... Thanks again to BV and others who posted their advice!