So I've recently run into an aging custom built gaming rig (about 4 years old I think) that seems to be in its death throes.
Up until a couple of hours ago, it was running Linux Mint just fine. However odd things like settings not saving and the wallpaper not loading started to happen and a few reboots later everything went pear shaped. After that, booting from a live-usb worked for a time before that too was rejected as mainstream.
Booting from the HD or a live-usb of Mint 12 results in hanging at 'Boot from CD :' at startup. Oddly, I can boot from a CD
The PSU. After coming falling into my hands I replaced the aging PSU (that was making horrible noises) with a brand new Seasonic M12II 620W (the same wattage as the previous one). All was well until connecting an old SATA HD and turning it on, at which point the HD started smoking. Since that HD already made terrible noises too, I wrote it off as either a HD problem or that the power to the port on the PSU (as it's partially modular) I was using had been wired incorrectly. Note that this is a different HD to the one currently 'running' the OS. The PSU's fan spins fine, and it did seem to power the rest of the system without any sparks, fires or blackouts.
The HD. An attempt to format the HD from the live-usb resulted in 'daemon is inhibited' errors. It seems unlikely that this is the HD's fault but it might be worth a mention.
The Motherboard. I can still access the BIOS, but with all the different problems there could be an issue here.
Is the PC keeping time correctly? Could be the MB battery is getting weak (4 years old or so) and the BIOS is "forgetting" its settings?
Flippin' hard to plug in connectors to the HD the wrong way to cause a HD cremation but can you verify the polarity of the PSU wiring? Just in case one of the connectors is backwards? Though I doubt this myself as the problem.
Could be that you have a few unrelated gremlins coming together for a party in the PC all at the same time. And you get the fun of cleaning up after they party is over
edit: At least as a first step verify all the BIOS settings match the HD configuration, CPU etc
Also I wouldn't be surprised if it turns out to be a problem with half the components. The old PSU was making such horrible noises that I wouldn't be surprised if power factors did terrible things to the rest of the box.
Are there any tools you'd recommend for testing individual components that I can download on to (or come with) a live-cd?
I'm coming up with nothing more to suggest other than go back to the minimum setup MB, CPU, 1 stick ram and an empty HD (one that you can borrow from somewhere thus saving your existing formatted drive). Double check all connections (both cable and slot types).
See if you can get it to start to the point where the message "no operating system found". That should help eliminate the hardware problems.