I've just had a nightmare after stopping a 12 hour optimisation of my dual G5 using Tech Tool Pro.
Drive wouldn't mount and error was invalid node sector.
Thankfully Disk Warrior came to the rescue and mounted the drive and allowed about 95% data rescue.
I'm not into bagging software but as I've noted on other forums be REALLY REALLY wary of using the optimisation tool in Tech Tool Pro. Good software for mac maintenance but left me with a few heart-stopping moments.
The Apple tech reports it may also be a hard drive failure which didn't come up in my checks...
::: Time flies like an arrow but fruit flies like a banana :::