|The hard drive must have an operating system installed on it in order for the hard drive to be detected as bootable by the mboard's bios.|
Does it ?
If it does...
Go into your bios Setup and make sure the Boot Order or similar settings are right.
See Response 1:
Some computer's bioses will not boot the hard drive and will give you that error if you have a non bootable USB flash drive plugged in while booting.