Having just backed up to a second internal drive on my G4(AGP), I figured, what the heck, and allowed this update to download via software update and run itself. On reboot I get an open firmware screen. nothing. wait... type "mac-boot" and I get a flashing question mark icon... (no bootable drive found). more waiting, giving it lots of time (I mean, this is only a G4, which Apple touted as a supercomputer...) nothing. I tried a hard reboot and get dumped into open firmware again. So I rebooted with zapping the PRAM (cmd-option-P-R) and then it booted fine. I checked Apple's Knowledgebase for "Hard Drive Update" and found zilch. Needless to say I decided not to run this update again until I find out more from an Apple Systems Engineer with access to better tech databases than I have.