Line through a circle symbol, can't find a way around it! Help please!

Question

On start up of my MDD G4 I am getting a no entry symbol, a circle with a line through it.
To give the full background of events, 2 evenings ago the Mac froze up for no reason, at the point I think where the screensaver would’ve activated due to inactivity. There was only mail and firefox running at the time, no videos or anything – this error, or at least the freezing of the Mac (no movement even from the mouse) repeated itself later on that night and yesterday evening as well, although the next evening the Mac wasn’t on long enough for the screensaver to start up. On those occasions the Mac restarted fine. Last night however I got the circle symbol but was able to hold down option key and select my previous drive to start up with, I ran Diskwarrior and it found a few faults with my main larger drive and repaired them and the Mac restarted with the larger drive as it’s startup drive absolutely fine. I put the Mac to sleep last thing at night, however this morning it failed to wake – note that the power lights on the Mac and Apple Monitor were glowing and fading fine as they should.
When I restarted the Mac via the power button on the Mac it started up fine, I had the Mac symbol with the spinning icon (not sure how you describe it!) as normal, however the fans then slowed down as did the hard drive noise and that’s when the circle with the line through symbol appeared.
So far I have tried disconnecting all peripherals from the Mac, resetting the PRAM, resetting the CUDA switch on the motherboard, starting up holding X and starting up from my Diskwarrior CD – unfortunately even the startup using the Diskwarrior CD fails to bring up a desktop of any sort and instead brings up the circle with a line through it.
Can anyone shed anymore light on potential problems or more importantly solutions as I’m tearing my hair out on this one!!
As further info, there are 2 other additional hard drives in my G4, one of which was my main startup drive for around the last year (a 500Gb drive connected via PCI card), which has worked flawlessly until now. I have tried starting up with this drive disconnected but to no avail.
One more piece of information which may help is that if I hold down the option key at startup the Mac does see both OSX system on my original and additional hard drives (and Diskwarrior CD if inserted) – however no matter which one I select it goes no further than the line through a circle symbol.
Many virtual drinks for anyone who can solve this!!!!!!!

Answer 1

One of the Level 4's, the Hatter (I hope he stops by personally) gave me the following insight:
The hatter wrote:
A really badly corrupt directory can prevent booting from any drive or media, because they all look to see what other boot volumes are out there.
With G4/5's the best way to clear corrupt nvram was to boot into Open Firmware and reset nvram and a good idea to do when swapping drives, memory, PCI cards.
Try booting after removing or unplugging the hard drive. May need to pull it and put in FW case to recover anything.
So what could have happened when you crashed is that there was disk damage, probably to the disk directory. If that happened say to a single HD, then according to the Hatter, you won't be able to boot from any drive since there's a kind of checking to see what other drives are boot volumes out. That check could be failing in your case because of the directory corruption.
You have two HDs in your MDD, so either one or both could have suffered directory corruption.
If I understood one of your posts, you disconnected the SATA drive, and still saw problems. If so, perhaps your old HD suffered corruption.
So you might try disconnecting the old drive -- be sure to disconnect the data cable because your Mac will can cause fits if the data cable is connected to an unpowered HD. Then try to boot from the Install CD or even Diskwarrior. I'm pretty sure you can boot from the optical drive even if there aren't any HDs.
If you can boot from CD with no HDs, then we've got it narrowed down to directory corruption. Thereafter, you might see if you attach only the SATA if you can boot to diagnose the just that drive. If you can, that's great because it would mean your backup HD suffered the damage and not your main drive.
As indicated by the Hatter, if you've got damage to the SATA, it may mean your best chance is to connect it externally. That way you can boot, thereafter connect and diagnose.