Steam – Download Portal 2 authoring tools without Portal 2 installed

I would let Steam download whatever it wants to right now and reach a stable point.

(or you could rollback from your Time Machine backup).

When you have something that works, then consider asking Steam to back up your files to external media so you have working files next time you want to restore something.

In my experience with upgrading operating systems, putting the contents of the games folder in the (in your case) windows 7 steam folder, into the windows 8 one, launching through steam works, as long as the files are put in the correct path. It will appear in your library as uninstalled, however when you launch then it'll 'validate' and be ready to play in, at most, 5-10 seconds.