I would advise just installing WIndows 7 to a VHD file, store that on your MicroServer then just add that to the Windows BCD bootloader.
That way, you can just boot into Windows 7 whenever you wish regardless of the external HDD.
If you want something like you are describing:
- Plug in USB HDD
- Set BIOS to boot off of Internal HD
- Plug in USB stick
- Deploy Windows bootloader (Grab EasyBCD) onto the USB stick
- Add Internal HDD and USB HDD
- (Whilst editing USB stick BCD) Change BCD order for USB HDD first, internal second.
Now if it boots off the internal HDD, it should load the default OS.
If it boots off the USB stick with the USB HDD connected, it will boot off the USB HDD.