Suddenly, without any apparent reason, neither of my 2 hard drives are detected during boot, sometimes. Meaning... they weren't detected using auto detect. I changed BIOS settings to USR and put in the numbers, still not detected. Switched back to auto detect. I figured it might be the cable, so I put in a brand new one. On first boot, they were detected, everything was fine. Yippee, problem solved. Nope. Rebooted, again not detected. Jiggled things around, they were detected. Plugged in my other drives (CD, floppy) and they (the hard drives) weren't detected. Unplugged the other drives, still not detected. Removed/replaced the hard drive connector, they were detected. I don't see any bent pins. I don't know if my jiggling/replacing had any real effect -- might have just been coincidence, since sometimes that method was not effective. Right now I am using this system while it detected the drives, so the drives are fine. I've had these 2 drives working for at least 6 months and haven't made any changes inside the chassis recently.
If it isn't the cable or the connector, and it isn't the drives or the BIOS, what could it be?