I have Debian installed on a External HD and I want to take it to another computer. When I hook it up and boot into it, it says that sdb1 cannot be found or something. can I change the partition it thinks it on so I can boot into it on another computer?

Where is the bootloader installed? (I infer that it is in the MBR of the external, and that you have set the BIOS to boot the external drive)
The bootloader gets setup to look for its config file in a certain place--are you getting past the point of seeing a bootloader menu?

Also, when it goes looking for sdb1, there may be no such thing on that computer---it depends on what else is installed.

Sooo...tell us what the configuration is and exactly where in the boot process it is getting stuck.

The link below will lead to an article on booting which might be helpful.

You would need to modify the /boot/grub/menu.lst and /etc/fstab files to reflect the new drive location as seen by the computer you are on using something like a Knoppix disk to boot with first to see how the drive is seen then mounting it and editing the files so do it.

Ok, well, I get into GRUB (which is on the external) just fine. It starts to load and I wait a couple of minutes, then it gives me a error saying it couldn't fine that partition. Debian also thinks that Both my CD Drives are hda and hdb, then it thinks my main hd is sda then my external which is sdb. So I don't know if that has anything to do with it or not.

Make sure that you do not have any other device installed to external USB ports.

Edit /boot/grub/device.map as follows:

(hd0) /dev/sda

Boot with the external USB hard disk again.

In addition to editing /boot/grub/menu.lst and /etc/fstab, you may also need to rebuild the initrd ramdisk image to remove (or change) the RESUME swap partition. If you don't know what "resume from swap" is, then you don't need it and you won't miss it.