Linux Mint Forums

?
You say external drive, but not a USB drive
What do you have?
e-SATA
Firewire
something completely different?
Without knowing that I can't be sure, but this may be the solution for you
http://www.linuxmint.com/wiki/index.php ... annot_boot
This is provided you connect and disconnect the external drive

Ouch
I realize now that USB disks need to be mentioned
From your description I guess that is your problem