Original OS & Backup won't boot (HUGE fan noise)

[super8trash]

Since I got SuperDuper! I've always been starting up from my 'Sandbox' so I could try out new installs. On a 'LaCie d2' I've been backing up 'all files' through a 'smart update'. Works great!
So I have three startup disks: original OS, Sandbox and a 'LaCie OSX' Backup: 10.4.4.
Only yesterday I tried for the first time to start up from the original OS X partition and from the LaCie system. To no avail. The grey startup screen keeps hanging and after a minute or so there's this huge fan noise that won't stop unless I push the power button.
I searched around a bit and heard of extreme G5 fan noise on 10.4.3 but I can't 'connect' this problem with my 'boot problem'.
So before reformatting and reinstalling anything, any insight much appreciated...

Tom
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G5, 2x2 GHz