I have an A7N8X-DX ver 1.04 mobo, and just bought a samsung 1 TB Sata HDD. So for right now I was just going to use it for a slave till i get my new system going. So I have a 40gig maxtor drive as my os drive. I can not boot with the samsung drive plugged in, it hangs up when it finds it. I can boot with it unplugged, plug it in and see it fine. I have update my bios and sata conrtoller to the latest version I could find on Asus's site. I have tried every combination possible of bios setting HDD positions and so forth i can think of, but can not get it to boot. I even tried having only the SATA drive plugged in and set the bios to boot from floppy, it can't even get far enough to boot off the floppy drive.
Sorry if I double posted... can't find my other post
It seems that others have had similar problems with that MB: http://www.tomshardware.com/forum/150214-30-a7n8x-faile...
In fairness, it IS an obsolete system that dates from the first days of SATA, so modern SATA drives might be expected to have some issues with it. If there is a jumper on your drive to force it to use 150MB/s data transfer mode, be sure to try that.