Connect an eSATA External Hard Drive directly to Logic Board?

I have a Power Mac G5 DP1.8GHz that won't boot from any any SATA drive installed in its two bays. I thought I would see if it would boot from a FireWire drive or from a SATA drive installed in an external enclosure with both USB 2.0 and eSATA connectors. The eSATA cable, however seems to correspond to nothing on the logic board. What am I missing? If the built-in SATA controller is bad, would it be possible to boot from an external eSATA device that is connected to a SATA port on the logic board? I know this seems obtuse but I am in the end stages of trouble-shooting and have lost all sense of proportion.