So I want to reinstall Shogun 2. I have a retail copy. I try to install it. But it keeps wanting to download the whole game from the steam server.

I go through the tech support. It says a bunch of steps (in the official help list). I do them 5 times. Still nothing.

What the hell is going on here? Why the hell is it refusing to recognize my goddamn discs?

not when i have to download 13 gigabytes. it takes too damn long and sucks up too much band.

I had similar problems with Modern Warfare 2 back in the days.
Steam wanted me to download 14GB even though I had both discs in my tray already.

Try deleting everything Steam has downloaded so far and try to install it again.

that would completely defeat the purpose of me trying to save time by installing the game from the disc. if i did that, i would be spending even more time reinstalling every single game i ever got.

lol sry I meant you should delete all Shogun 2 files that Steam already downloaded on your HDD.

Yeah, I also had to download Portal2 after installing it from disk. Really annoying and absolutely makes no sense.

https://support.steampowered.com/kb_article.php?ref=5357-FSQM-0382

Follow these exact steps. Make sure Disc 1 is in as well. As you go through the steps at the end it may still showing you as "downloading" -- you're not, it's just how it installs from the disc,

Shogun 2 is a large game with numerous patches, so there is also a chance that you'll still have several gigs to download once the game finishes installing from disc.

When it installs from the disk it still says downloading it just installs way faster. Deus ex, dungeon siege 3 both did this.

Then it automatically downloads patches unless you turn it off (which is annoying since some games are more movable with different patches),

I have a Mac.. So I know why steam is a pain for me

It's probably got to do with the files on the disks being out of date. One time I downloaded Portal 2, didn't play it for months, then when I went to play it had to convert and re-download almost all the files to complete a major performance update.

There are probably two ways to get around this.
1- You could try selecting "Do not keep my games up to date" from the Steam settings, this should keep it from looking for new files when it's installing.
2- Another possible work around may be to set Steam to offline mode while you install so it can't look at the files on the server (If that's possible).

If you do either of these two options, when you turn updates back on or go back to online mode you may once again be hit with the problem of the game re-downloading or converting to Steams newer format.
I also had this issue with Half Life 2 from the Orange Box DVD's when I last installed it, if the game you're installing isn't played online you may choose to just leave updates turned off for that specific game (if that's possible).