I want to know this, because I deleted some ncf/gcf files for portal 2, because the updating was stuck. And now it wants to download it all (11GB) again, so it would be nice if I could copy the content to the download location, without Steam noticing.
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.
Game files are stored in
Copying and pasting files there might work as expected but - as "The Annoying Pyro" mentions above - is prone to (existing) file corruption. So I would recommend the following procedure:
Please note that this might not work as well as I might look in the first place. Steam might want to download all files again when it thinks they are corrupt. But it's worth a try.