Game license and swapping consoles

If the game is already downloaded on the external hard drive. You should be able to just plug it in, buy the game and start playing.

You would not need to do a license transfer if you're just swapping hard drives. That's only needed if you swap consoles.

I believe Farscape247 is referring to the comment "they have to log into my account". That is not allowed. Accounts cannot be shared.

What you described with home console swapping isn't explicitly listed as a usage scenario, but it doesn't violate the license agreement (one Home Console license, one gamertag license).

Crunchy:

Xbox content licenses are bound to two things. The gamertag that purchases it, and the console it's purchased on. Transferring licenses just swaps the licenses over to the console you perform the transfer on. As long as one of the two is present - you are
either on the console who currently has the licenses, or signed into the gamertag that purchased the content - you will have no problems playing it. If you do run into issues, call customer support to set up a manual content license transfer.

As long as the hard drives are for the same type of console (slim to slim, fat to fat) you can swap them without any issues. You won't have to perform a license transfer each time you swap the hard drives, but if you had purchased anything on the 2nd console
and not the one you're keeping, you'll need to perform a license transfer and redownload the content in order for that content to work properly. Any games that were installed from disc will need to be re-installed though.

There is not a connecter to plug the hard drive in via USB.