I put together a new computer this year. The hard drive, however, is not always seen by bios. Sometimes it is, more often it isn't. I've tried replacing various components such as the motherboard, psu, and dvd drive but the same problem persists. I've run Seatools since the drive is a Seagate. It always passes.
Most recently I've borrowed another hard drive to see if the problem occurs. This is day two, and it hasn't yet had the issue.
Bios isn't seeing my original hard drive (I have both connected) but when I boot the computer with the borrowed drive I can see and access mine in "My Computer."
Both drives have my copy of Windows 7 64 installed on them, so both can boot. Just a few days ago I did boot from my original drive, and bios saw it.
Asus p8z68-v lx 1155
Seagate Barracuda st2000dm001 2TB
LG 22x dvd burner
intel core i5-2400 sandy bridge 3.1 ghz
seasonic m12II 620 bronze
gigabyte gv-4777d5 1gd radeon hd 7770
8 gb patriot ram