IDE (or PATA) Emulation Mode is an option in setting the Port Mode for a SATA drive. When you choose that, the BIOS makes the actual SATA drive appear to Windows to be a plain old IDE drive. This means it works just fine, EXCEPT that it does NOT make use of some advanced features of real SATA drives like NCQ and a few others. The mode is especially useful if you install an older OS that does not have built-in drivers for SATA or AHCI devices. This includes all versions of Windows up to and including Win XP; however, Vista and Win 7 both do NOT need this setting because they DO have built-in AHCI drivers.
1. If you are using this drive as your boot drive with Win XP or earlier as your OS, you have two choices. One is to set the BIOS to IDE Emulation Mode, then install the OS normally. The other is to set the AHCI mode and use the "F6" key system when prompted early in the Install process to install the required AHCI driver from a floppy disk.
2. If you are using Win XP but this new HDD is NOT your boot device, you can install it and configure the BIOS SATA Port Mode to AHCI. Then you must install the AHCI driver in Windows XP, probably from the CD that came with your mobo or from the ASUS website.
3. If you are using Vista or Win 7 you should set the mode in BIOS to AHCI.
4. If you already installed and Partitioned / Formatted the new drive with the BIOS SATA Port Mode set to IDE (or PATA) Emulation, then you change it to AHCI, you may not be able to access the drive's contents. (Sometimes it works, sometimes not, according to confusing posts around here.) If that happens you should set it back to IDE Emulation mode and back up all its contents to another drive (if it has files). Then reset the Mode to AHCI and use Disk Management to Delete its Partitions, then Create new Partition(s) and Format them with the new Mode setting, then restore files to it.