My iMac has been very unstable lately, frequent lockups. I had to turn off the computer by holding in the power button a couple of times. Sometimes it starts up and other times it just sits at a blue screen. My Time Machine external FireWire drive may be failing. Sometimes it does not mount. It shows up in Disk Utility and I have tried the repair function and then finally reformatted it. Thought everything was fix and after Time Machine made a new backup I entered Time Machine and the computer locked up again. Shut off the computer again, and again it started up and stop at a blue screen, restarted it again and it fully started. Again the Time Machine drives does not mount. Also a portable USB drive will not mount, I get and error about not being properly unmounted. The USB drive works fine in my MacBook. What would cause the iMac to not mount any external drives? Can a failed drive that does not get unmounted correctly corrupt the iMac to the point it can't mount any external drive?