The computer may have USB Mass Storage support in the BIOS. It
may be trying to boot from that drive, and failing. By
disconnecting the drive, the BIOS no longer trips over that
drive, during startup.
I've also heard of computers, getting stuck on the 9-in-1 USB
card reader, for a similar reason. Disconnecting the card reader,
magically allows the computer to boot.
Is there a fix for this ? I haven't read of an account, of a
way to modify the contents of the external hard drive, so the
BIOS won't poke at it. But maybe someone has figured it out.
It could be, that the partition is marked as "active", and
maybe that has something to do with it.
http://forums.seagate.com/stx/board/message?board.id=onetouch&thread.id=606
Just a guess,
Paul