If you have enough HD space you can just copy your portal 2 game folder from your pc onto your mac, at least it will save you the time downloading it :)

If you have all your games in a new library on different HDD than it will work. After you install steam just add a new library and select the one on your another HDD. Steam will check the files and update them if necessary. However I didn't have any luck transferring games stored in my steam folder in program files. Moving or deleting ClientRegistry.blob didn't help then. So next time I would transfer all games from default location to a new library before reinstall so I can just copy the library over later.