I have a P4SPA+ / P4SPE motherboard with 2 IDE hard drives, 1 IDE CDROM and 1 SATA hard drive connected to it. The SATA is configured as AUTO inside BIOS, but it is unable to boot from the tertiary SATA drive.
Whenever device is in a tertiary or quad channel, it cannot be used as a boot drive, but it can be accessed in XP. To use it as a boot drive, you should set the SATA to as the primary or secondary channel.