Forbidden Screen - Error: "Still waiting for root device"

Question

Greetings to all! I come to you with my boot troubles as a supplicant, in the hope that someone can help.
I have a PowerBook G4 15-inch, pre-Intel from late 2005. Until today it always worked and rarely hanged. All I've ever needed to do when in trouble was reboot. I've updated the Apple OSX software regularly and have never touched the hardware, except once 18 months ago to add memory. All I do with this machine is use it. It's been weeks since I installed any new software.
There were no warning signs before the problem started today.
FIRST SIGN OF TROUBLE: The machine was unusually slow, showing beach balls on every minor move. I powered down conventionally and later turned it on again. After a minute I got the "Forbidden Sign" grey screen, which I had never seen before. So I rebooted, and got that again.
+I turned to the only other available machine (very old PC, unfortunately) to browse for solutions, and have so far tried the following:+
1) BOOT FROM INSTALLATION DISK: Forbidden Screen. After a couple of attempts I removed the disk (by clicking trackpad on power up). At some point, in desperation, I even tried Disc 2.
2) RESET PRAM (Command-Option-P-R on power up) : I got three chimes, then the grey screen. It stayed on the apple and gear for many minutes. At least it never went to the Forbidden Screen.
3) FILE SYSTEM CHECK (Option-S on power up) : After a few steps it shows error messages (see below) and starts repeating, "Still waiting for root device," over and over.
4) RESET NVRAM (Command-Option-O-F on power up) : I got a blue screen with text and was able to input "reset-nvram" (response: OK) and then "boot." Result: Forbidden Screen.
5) SAFE START (Option on power up) : Forget it. Forbidden Screen. Same when I try it with "C" as some sites recommend. Same with or without the Install Disk in the DVD driver.
6) POWER MANAGEMENT UNIT RESET (Shift-Control-Option-Power): Subsequent boot still produces Forbidden Screen.
*THE ERROR MESSAGES*
Here are some of the messages when I start up with Option-S:
CSRHIDTransitionDriver::probe booting in single user .. do not match
Extension "com.apple.driver.AppleUSBTCKeyboard" has no kernel dependency.
Extension "com.apple.driver.AppleUSBTCKeyEventDriver" has no kernel dependency.
Apple PMU::CLOCK RESET! PMU WAS PROBABLY RESET SOMEHOW!! +(Yes, I did that, but it wasn't showing this before I did, and it WAS showing all of the following.)+
Security auditing service present
BSM auditing present
disabled
rooting via boot-uuid from /chosen: +(I think the long code that follows is the serial number?)+
Waiting on <dict ID="o"><key>IOProviderClass</key><string ID="1">IOResources</string><key>IOResourceMatch</key><string ID="2">boot-uuid-media</string></dict>
PMUMS::start ERROR: failed to find IOATABlockStorageDriver
Still waiting for root device
Still waiting for root device
Still waiting for root device
+(etc. ad infinitum)+
*So now I'm begging anyone for answers to these questions:*
Am I doomed?
Does anyone know what the problem is?
This seems to be happening prior to disk access, as it happens with or without the Install Disk. So do I still have the hard drive working? Is my data on it?!
Was resetting the Power Management Unit a bad idea?
What can I do next?
THANK YOU THANK YOU THANK YOU!
Nicholas from NYC

Answer 1

Thanks for your reply, Louie!
In the meantime I have succeeded in booting via the Install Disc using Option on power up. The Hardware test shows nada wrong. Disk Util repair goes up to "Rebuilding Category B-Tree" and then fails. Frustratingly, the HD is somehow visible, because the diagnostics correctly identify it and give correct values for empty space. Right now I'm running the Erase Free Space option and it's working - how is that possible? I've gotten into Single User mode and tried various FSCK commands. Sometimes it comes back with the superblock error and "Bad Magic Number" which I believe indicates a corrupted block within the boot sector. I can't figure out how to reassign without reformatting and losing the data. This will cost me a few weeks of data, but really not that much work - still set me back by days.
Next step will be to start taking it apart, as you suggest, starting with the easy matter of trying with just one DIMM or switching the DIMMs.
Thanks again, and if anyone knows how to help - how does one reassign a superblock from the Single User mode?