It does not show the drive in the BIOS, or are you talking about Windows?
If you are talking about Windows, you have to format the drive, assign a drive letter, and set the partition to active under Disk Management in Windows for the drive to show up as a usable drive.
If you were not running a SATA drive before, the SATA controller may not even be turned on. Check on that. It will find the drive and display it during POST no matter what mode you have it set to, AHCI, IDE, or RAID. But there are circumstances that must be dealt with if you choose AHCI, or RAID before you install the OS if you are running XP. If your are running Vista, then you are okay.
If you want to boot to the SATA drive, you will also have to put it into your boot order.