GRUB Error 21 after install

[<wednspecial@verizon.net>]

I installed Debian onto a usb hard drive (sda) using a laptop that already had Debian installed.  Now when I try to boot up the laptop, I get the following error:

GRUB Loading stage1.5
GRUB loading, please wait...
Error 21

I know that Error 21 means that GRUB can't find the disk drive.  The menu.lst file only refers to the hard drive on the laptop (hda).

When I use a Knoppix CD to look at the laptop, the Linux partition is still on hda2.

I get this error whether or not the usb drive is attached.

How do I get the laptop to boot again?