Hello and welcome to Bleepingcomputer.
I see where you said you made sure the SATA cables were firmly seated into the SATA connectors.
The fact that cycling the computer seems to resolve the issue is an indication that there is an intermittent connection, or there is a BIOS setting that is not correct.
How's the life of the CMOS battery? Is it a new one? Could it be weak or dead?
I would recommend your paying closer attention the settings you have in the BIOS, there might be one thing there that is not set correctly.
Make sure the drive is detected and the drive is set as a boot device in the boot menu, make sure you save all those settings by saving to CMOS upon exit.
Make sure the cable to the hard drive is not damaged where the wires go into the plug.
Make sure no corrosion, or a speck of dust has come between the connections, trust me this does happen and sometimes isn't noticeable.
I am sure its a simple little thing that's being over looked, we just have to work with it till it is found and corrected.
Edited by MrBruce1959, 11 July 2010 - 06:57 PM.