In summary, Thanks to murphychris, here is what I have so far, To be able to install and run Windows 7 off of a Thunderbolt solution (Seagate/buffalo/etc), a FAT partition needs to be created on the internal SSD. Per murphychris, for the rMBP the internal partition is needed by mac to create hybrid MBR and sets up the internal drive to take the bootloader from the windows installer. The first solution is to use boocamp assistant utility wants to create a 20GB partition. I used a different method to create a much smaller partition (mine was 5gb but likely can be as small as 1 GB). 1. run diskutil command: Code: diskutil list you will need to know if your internal SSD is disk0 or disk1 (in this guide it was disk0 2. Create the small partition: ***my ssd is a 512 GB drive, you can break down the partition here *** Code: diskutil resizevolume /dev/disk0s2 494G MS-DOS FAT 5G 3. Within Mac, create a single msdos partition on the external thunderbolt drive. (GUID or MBR) 4. for the rMBP, use the Bootcamp assistant utility to create your USB windows 7 installer from an iso and also with the bootcamp drivers. 5. Reboot and press the option key at the chime. 6. Choose the windows installer 7. at the drive screen of the installer you will see your 2 drives. drive0 maybe the external. Format the external drive ntfs and then install to the drive. 8. On reboot and eventually install bootcamp drivers. Trial and error: I tried install without internal partition and received 0x8030024 error which in some of apple forums can be overcome by removing the main drive. I did this but installer would freeze. Good luck. If you need any help let me know.