the fdisk -l report is:
Running the command as root: fdisk -l should provide us with your partition information and may be helpful. Also, that is a lower case Letter L in the command, not a one.
How do I install Grub to the mbr of the internal drive?
Genereally the default installation will install Grub to the mbr of the first drive. Since you are able to boot directly into windows without the external drive attached, that did not happen. You must have selected another choice. You will need to install Grub to the mbr of your internal drive. It would seem your Grub stage1 is looking on the wrong partition for stage1.5.