Don't have that mobo, so I'm not very familiar with the BIOS. If its any consolation, you really aren't losing much, if any performance going to ATA33. Storagereview shows that drive having a maximum STR of about 28 Mb/sec, which is 5 Mb/sec less than ATA33 can handle. You may lose some of the burst speed, which is not all that important anyway. So, until you figure out the mystery, don't sweat it. Just enjoy the new drive.