Oh my, that's a lot of data.
Steam updated my TS this morning and it was only around 20 MB.
Some guesses: Have you tried shutting down and restarting the Steam client? Also, in case you've installed TS in a SteamLibrary on a removable HD, it might be worth checking that Steam can still find that library. I've seen it before where it somehow forgot about them, probably after the external disc was disconnected for a while. Also if running on an external disc, check that the drive letter is still the same as it was when you installed/last used the game. Windows sometimes changes the drive letters when one dis-/reconnects several external drives and now maybe Steam has started a completely new installation on the disc that has inherited the drive letter from the one with the TS installation.
Cheers,
Michael