[dnanian]

Assuming the drive is properly partitioned (as "GUID" for an Intel Mac, or "APM" for a Power PC), try powering everything down and disconnecting the drive from the Mac and AC.

Get the Mac to the Option+Boot screen and wait for it to 'settle'. Then, plug the drive into AC and the Mac. Wait for it to spin up audibly, then press the Refresh (Circle-arrow) button. If it shows up, select and boot.