I have a copy of Debian installed on an ext3 partition of my 80gb externel Lacie drive, however, I can't boot it.

I installed GRUB using:

All of the /boot/grub files install onto /mnt/sda1 (a small vfat partition for boot) but when I boot into the harddrive (my BIOS supports this) it doesn't do anything, and simply skips to the next boot device.

My guessing is that the MBR is not being written to properly, but I have no idea how to access/view the MBR.

Am I simply not installing GRUB properly (although it gives me no errors) or am I doing something else wrong?

I'm not sure because I don't know your hard drive setup, but If you're saying that your boot partition is on your external drive then I suspect that it needs to be on your internal hard drive.