Can start using external USB drive, but not internal drive
Suddenly I got the "Folder ?" icon on start up. I plugged in a USB backup drive, restarted and after booting, it took 5 minutes for the keyboard to work so I could login in. The internal drive did not show up with disk utility.
I replaced the internal drive with a drive I know is good, got the "Folder ?" icon again. I plugged in the external USB drive, restarted, and again, had to wait about 5 minutes for the keyboard to work before I could login. Again, the internal drive does not appear in Disk Utility.
The computer runs fine (if a little slow) booting from the USB drive, so I am assuming it is the logic board? Or could it be a part that connects the internal drive to the logic board? So do I need to do other tests, replace the logic board, or replace a smaller part?
If you have a known good hard drive try it. If you don't, replace the cable that connects the hard dive to the logic board. If you have recently disturbed this cable double check it's connections. If replacing the cable does not solve the problem. You will need to replace the hard drive.
Normally when the hard drive or the hard drive controller on the motherboard are/is failing they give symptoms over time that give hint there will be a failure and don't just die. You have given no indication of previous problems to suspect a problem with them. Also, the cable is the cheapest part to replace.