Adding a boot partition to my external hard drive

Actually you can do.

Goto your Disk Management (Hope you know the Disk Manager) or Type DiskMgmt.msc in Run

Now you can see in the 1st Row Disk0 shows as Dark Blue Border, it means it is Primary Partition. and rest of them are in green border. it's called Extended Partition.

I guess your 2nd Partition is Extended. because you cannot boot from extended partition. all bootable partitions should be Primary.

1) All you have to do now is, open the DiskMamagement and Right Click on the 2nd Partition and Delete it. so it will become full black.(you need to delete twice)

2) now in the RUN type CMD (Command Prompt). and type again Diskpart Press Enter

3) in that, type List Disk that will show you the Volume list so select the appropriate Disk by typing Select Disk 0

4) now type again. Create Partition Primary

5) after that type again to format the created volume Format FS=NTFS Quick

and That's it. now Windows 7 DVD Tool will Detect your Partition