When you installed Ubuntu to the external, where did you install Grub? The root partition on the external? master boot record of the external? master boot record of the internal?
Grub Error 17:
Cannot mount selected partition
This error is returned if the partition requested exists, but the filesystem type cannot be recognized by GRUB
Grub Error 21:
Selected disk does not exist
This error is returned if the device part of a device- or full file name refers to a disk or BIOS device that is not present or not recognized by the BIOS in the system.
Can you load the Ubuntu install CD and open a terminal and run the command: sudo fdisk -l (lower case letter L) and post partition info here?
Which version of windows are you using?