Hold the Alt/Option key at Startup to invoke the All-in-ROM Startup Manager. This draws an Icon for each potentially-bootable drive.
Are any bootable Volumes there?
When all the Icons are drawn, the Eject/Open button on the keyboard goes "live" and can be used to open the drawer. If you insert a bootable DVD, it will re-scan and show its Icon, if potentially bootable.
The 10.6 Installer/Utilities DVD has Disk Utility on it. Answer only the "What Language" question and wait a quarter minute for the MenuBar to be drawn. Choose Disk Utility off a Menu. Look at the Hardware_name for each drive, anc check its SMART Status in the lower right of the window.
(Repair Disk) on as many as you can.
Thanks for the reply. I highly doubt it's a hard disk issue because my external USB drive also wasn't working as a bootup device (when I pressed option at startup and chose it). The computer was actually able to boot off the DVD, but it took a while. There was a LOT of noise coming from the DVD drive and GPU, and everything was very, very slow. I tried a different DVD (the product disc instead of the SL install DVD), and it was faster.
I tried repairing the disks. One of them was OK to begin with, and the other had a minor invalid block count that it fixed. But it still won't boot from either drive.
My guess at this point is that the GPU somehow has something to do with it. It doesn't work even with no monitor plugged in, but the GPU fan is running pretty fast when I plug the monitor in, and it would explain non-disk-related problems. I tried booting in verbose mode, but there was nothing useful there.
These run with "one size fits all" simple display driver:
• Regular Startup:
gray screen at startup (the dark gray Apple is loaded off a drive)
At the transition to "normal" boot login screen, the screen changes to "automatic" using the loaded Driver.
• Boot from the DVD
• Safe Mode (hold the Shift key at Startup)
everything -- The display Driver is not even loaded
So if you can boot into Safe Mode, this adds weight to the assertion that the display card is the problem.
Safe Mode loads a minimal set of Apple-only extensions, so other added extensions could also be the source of the problem.
If the Caps lock light operates normally, the software is still running.
Booting in Safe Mode did not help, but it did show a grey circle with a cross through it instead of an Apple logo like before.
I remembered that there's an Apple Hardware Test (a bit hard to figure out how to do on your specific machine ), and I booted into it. Before I even ran the test, it told me that there is an error. The error message was just the name of one of my RAM slots. So... a piece of RAM might be dead. I ran the extended test, and it was halfway through after 30 minutes, but I stopped it because I don't have enough time. I have to continue this tomorrow.
As for the GPU, its fans might just be running because the system runs them at a high level by default when it's in certain states. I know older Macs did this in Target Disk Mode with every fan, and the GPU fan was running during the AHT too. At least I hope this is the case. Right now, it seems beyond a hard drive problem. Maybe booting from a DVD works because it only uses 1 RAM slot or something. I'd much rather have a piece of inexpensive RAM be dead than my GPU, and besides, I've kinda wanted to upgrade the RAM anyway.