I have a possible solution
Your problem sounds like a problem that I've had for the last two days. Except I was building two systems with the exact same parts and ran into the issue with the second computer. Once again involving a GB motherboard and Seagate hard drive. I tried switching the hard drive into the working computer to see if it was a motherboard issue but got no result until I deduced it must be some faulty hard drive. But you'd think the BIOS would know better than to detect a faulty hard drive. I even managed to format the hard drive using the Win 7 Cd, no luck there.
I came across your site and countless others until I had a moment of clarity and realised the hard drive is not being recognised because it has no partition. Zilch, nada, nothing. I then immediately hopped onto the pc and entered the disk partion through the cmd line.
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Step 1 - 'Select Disk 0' (type LIST DISK to find the disk number)
Step 2 - 'Clean Disk 0'. I ran a clean to reset all the bits on the hard drive (it took 4 or so hours on a 1TB HDD, so you might want to try the other steps and come back to this if it doesnt work).
Step 3 - 'Create Partition Primary'. This will instantly create the partition for you. After which you will be able to see the drive in the list so that you're able to select it, and possibly to advanced actions on it (Format, New Partion, Delete etc). you may need to delete the partition while in the list and create a new one while on that menu.
Step 4 - Select the partition and install Win 7. WOOOOOOOOOOOOOOOOOOT.
Anyway I thought I'd share the love even though its 4am and I should really be getting to bed.
Let me know how you go with this.
Cheers,
Gonzord