[Uri’el-2012]

I have mine vaulted on a 750GB external USB, but the only computer I own that would be able to use them is the G5.

I'm guessing DiskWarrior will bring back your machine
unless you have a hardware disk problem.

Just be advised booting from the DVD will take a few long minutes.

Graph the directory; I'll bet it is 20 - 30 % disorganized.

Then Rebuild the directory; again this will take some time.

When done reboot holding the option key down; switching to your disk drive.

[Uri’el-2012]

Spinning beachball.

some other thoughts:

Have you installed additional memory modules ?

They sometimes need to placed in a specific sequence.