After booting the system with out the storage it works fine.
Make sure that USB devices are disabled in the boot sequence or are after the hard drive in boot order. I've also had some systems were USB devices would intermittently prevent the computer from entering post.
Run "sudo tail -f /var/log/messages" before plugging in the external storage. Is the device itself detected (i.e. /dev/sdb)?
It might be a problem with the file system. What is the file system used on the external storage device?