Neither do I.
Or can describe what is going on.
Before this happens, you should hear a startup chime. That chime indicates that the G5 passed its POST, or Power-On Self Test. After that chime, you should see a white screen with an Apple logo. At this stage, the firmware is directing the computer to load the booter file. If this is successful, then you will see the spinning gear icon beneath the Apple logo. The spinning gear icon indicates that the kernel and subsequent files are being loaded.
and in this stage it keeps for ages...
Strange indeed. When you power up your control room, are there external peripherals attached to the G5 that turn on with a single (or master) switch?
no.
What would happen if you turned on only the G5 tomorrow morning? When you force shut down and then restart, do the cooling fans of the G5 ever ramp their speed up before the screen takes on a blue background?
no.
When the system hangs at the point of the white screen with the Apple logo, I think of a hard drive problem. This could either be physical failure, or it could be file system corruption, as it would be unable to properly locate, let alone load, the booter file.
thats what i think too.
If you can, startup off of the Leopard DVD you used with this system, and choose Disk Utility from the Utilities menu once the DVD loads. You will want to verify, and if necessary, repair the Macintosh HD.
HD failure, maybe. Known OS problem? I doubt it. Apple's QA may be bad, but I don't think it has gotten that bad...yet.
Ah, Seagate. Wise choice.