Hey up..

 

this looks as if the MBR on your internal HDD was overwritten by the Grub boot loader, which in turn is pointing to the partition on your external HDD (most likely called /boot) and once your external HDD is removed you have no access to the boot partition containing the grub.conf file which in turn points to your XP install.

 

Now it has been a while since I have had to do this so please be careful and backup everything!!

 

you are right with the 2 options for fixing, the MBR for windows is probably your best bet, but 1st you will have to right a new MBR to your external HDD to allow you to continue to use Linux (the XP boot loader will not allow you to add Linux as a boot option)

 

Boot into Linux and run the following command:

 

grub-install /dev/hdXX           *where XX is your hard disk and partition info*

update-grub                            *scans ads and configures all kernel boot parameters*

 

This should write a MBR to your USB HDD, test this by choosing the USB device from the F12 Menu at boot.

 

once you are happy with this you will have to fix your XP install

 

 

Disconnect your USB HDD and boot any XP CD, allow it to go through as though you are going to install windows and you will see that you have the option of going into the recovery console, this will dump you into a DOS screen, from there you will be able to do a fixmbr which should restore a XP based MBR,

 

Jay