Using an eSATA connected EXTDrive as Startup Disk

I am moving out of the country in a few days, and as such need to ship my Power Mac in a shipping container. As this makes me nervous, I've acquired an external drive running 2 750GB drives in mirror mode on which to place a clone of my Power Mac. Additionally I'm trying to confirm that I can boot up that clone via the eSATA connection as the startup disk.

I have the external hard drive connected via eSATA through the expresscard34 adapter of my MacBookPro (Intel Core 2 Duo), running the clone of my PowerPC G5, and I've tried several attempts to target the eSATA connected drive as the new startup disk, but it always boots back up as usual with the MacBook's startup disk.

To make this more bizarre, I CAN target the external drive as the startup disk via the USB connection. However, this is undesirable because of the speed, and the fact that I have a possible eSATA connection instead.

Any help on if this is resolvable, and if so how, would be much appreciated.