Heh, I'm afraid that involving that portable drive in any form or way is going to be a
complete nightmare for most kinds of development
You really really really want the two systems to be set up completely independently, able to run with only what's on the box and no dependency on an external drive that may or not be there or have been modified elsewhere. The thing is - source control in particular doesn't like surprises, and likes knowing exactly what is going on. Moving things around or possibly letting them be modified elsewhere (on a different box via portable drive) tends to cause all kinds of problems.
My strong recommendation is to install all the software separately on the two machines (or just one to begin with until you get everything working), and make sure no references remain on either machine to locations on the portable drive. The one thing that can tie them together is GitHub - both machines are set up separately, but point to the same remote repo and use the same SSH keys (after you go through the SSH steps on one machine you copy them to the other machine - that way you use the same key on both machines)
But even that would be a pain vs. simply just using one machine. Each time you switch machine you'd need to pull the latest from your remote repo first and get ready on the other machine. It sounds tempting to keep something on the portable drive, but while that might be possible it would be
very tricky, especially if you're unfamiliar with the concepts
The Git dir on the PATH likely has to be the CMD directory like in my example above. And again, the PATH is specific to each machine, so if you moved from one to the other it might not work the same.