I've had some different ASUS boards, but it is more general: Onboard RAID is not hardware RAID, it is only hardware assisted RAID. The main IO transfer goes over the normal SATA bus, the normal main RAM is used (by the driver), the CPU calculates the parity/runs the driver. SO you only have RAID it you've got a driver for your system. If the driver screws up, you're hosed.

BIOS modes:

• IDE: standard IDE compliant mode, to have any OS be able to talk to your disks. No RAID.
• AHCI: AHCI mode. No RAID. At least on every board I used in the last years same performance as with IDE mode. Sometimes worse drivers (blue/black screen). Supposedly supports hot plug (don't use it in servers, so can't tell)
• RAID: only RAID mode, configure in BIOS, probably monitor in Windows. This is also an AHCI mode.

You can't configure RAID while keeping data on those disks.

I've had some different ASUS boards, but it is more general: Onboard RAID is not hardware RAID, it is only hardware assisted software RAID. The main IO transfer goes over the normal SATA bus, the normal main RAM is used (by the driver), the CPU calculates the parity/runs the driver. SO you only have RAID it you've got a driver for your system. If the driver screws up, you're hosed.

BIOS modes:

• IDE: standard IDE compliant mode, to have any OS be able to talk to your disks. No RAID.
• AHCI: AHCI mode. No RAID. At least on every board I used in the last years same performance as with IDE mode. Sometimes worse drivers (blue/black screen). Supposedly supports hot plug (don't use it in servers, so can't tell)
• RAID: only RAID mode, configure in BIOS, probably monitor in Windows. This is also an AHCI mode.

You can't configure RAID while keeping data on those disks.