During boot up, the M/B gives error message that it cannot find a bootable device. I go into bios, manually select the HD with OS, then system then boots up normally. Once booted up, the computer functions normally. But next time I boot up, the same error happens again. I have tried clearing CMOS. I have tried disabling all HDs, CD drives, USB drives as bootable devices, EXCEPT the single bootable HD with OS. The M/B should see one and only one bootable HD. As I previously said, I built two other systems with same M/B, CPU, and main HD. But the other two systems do not have this problem. Any suggestions will be greatly appreciated.