RAID 1 on ASUS P5LD2

Greetings.

Set up Win 2000 Pro on ASUS P5LD2 board using SATA drive.

Wanted to set up RAID 1 mirror for it. Added 2nd drive. Went to BIOS, selected RAID, went into RAID setup.

It says that once I’ve selected my 2 drives for the mirror, DATA WILL BE DELEED !! Do they mean only that new 2nd drive, or will it erase BOTH drives ? ( Want to keep the data on first drive)

Thanks in advance.

Rick