I;ve had Steam and my games in general on a separate drive to my windows drive (in case it buggers up and I have to wipe the windows drive). When I've reformatted I've done one of two things
1. Do a fresh install of steam, then replaced the newly installed Steam folder with the old one (deleting the NEW folder first). sometimes steam itself;s updates but that's about it,
2. do a new install of steam directly over the old one , this works to but sometimes its has to redownload some games for some reason. But i would not do this tbh as you run the risk if old and new files getting into fights with each other.
I've not done 1 or 2 since the new Steam so it might react diffidently now, regardless I've not had to do a full re-download of everything in steam since just before Half Life 2 ep1 lol.
hope this helps.