New hdd not recognized

i have an old asus p5ad2-e premium lga 775 motherboard and a 40gb IDE hard drive recently i have purchased a new 500gb wd caviar black sata drive but when i put it in it just came up with the single 40gb drive and did not display or recognize the 500gb hard drive, what should i do?

P.S i dont know what Bios version i have i just thought id mention it i dont know if it matters, if you need more info just ask

Did you try changing the HDD mode in the BIOS from AHCI/ RAID to IDE ?

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.