Is the USB drive you're trying to work with USB bus powered or does it have an external power supply?
Quoted from this support document.
Good idea, but (sorry) I wonder why the HDD would show up under Mac OS X and the Startup Disk preferences, when it needs to be powered via another USB or power cable, which might or might not be present during the boot up?
the one i use is from old laptop, 2.5". gets power from usb, there is no external power supply.
How old? Is it P-ATA (IDE) or S-ATA?

And to all the others that might follow this thread, does the interface have any relevance on the booting abilities?