I uninstalled and re-installed Steam. I didn't uninstall my games, and they still exist on my hard drive (I checked).
I have an SSD and a HDD, and to save space I install programs (including Steam) to C:\Program Files (x86)\Steam drive (SSD) and games to D:\Games\SteamApps (HDD). How to I let Steam know the programs are installed there?
There is now an official support article at support.steampowered.com covering some other solutions as well.
Go to Steam > Settings > Downloads tab > Steam library folders. Add the D:\Games folder there and restart Steam. Steam should then be able to find the installed games again.
steamapps in it. Instead of selecting the drive letter only, you may need to scroll down to "Let me choose another location".
Commented
Dec 29, 2021 at 2:15
I know this is an old question with old answers, but I came across it when I made the mistake of backing up only my .\SteamApps\Common folder, and not the app manifests that accompanied them before uninstalling, deleting and reinstalling Steam.
None of the easy methods worked for me, and faced with the task of manually clicking through the install dialogue and waiting for it to discover existing files 1000 times over, I wrote a PowerShell script to do it for me.
It saved me a lot of time and effort, so I thought I'd share for anyone else who lands here.
https://github.com/ChiefIntegrator/Steam-GetOnTop
Yes, just click install and this will happen: 
After this it either installs the files or just does nothing. I had the same problem but I think it got patched by Valve a while ago
I can confirm the possible ways for you because I have done it many times.
Method 1: Moving games to a new hard drive (like SSD)
The root folder can be anywhere, but the games must be in the subdirectory named "steamapps\common" from the root folder. If you want the games to reside in D:\Games then you should create the folder structure: D:\Games\steamapps\common
The resulting game folders will look like this:
D:\Games\steamapps\common\Arma 2
D:\Games\steamapps\common\Arma 2 Operation Arrowhead
D:\Games\steamapps\common\Mountblade Warband
D:\Games\steamapps\common\Assassin's Creed IV Black Flag
Once the games are in place, in the Steam client open the Steam menu (top left corner) and select Settings. Open the "Downloads" section and at the very top, the first setting is named Content Libraries with a button Steam Library Folders. Click the button to open Steam Library Folders. Click the "Add Library Folder" button to add a new library then select the folder D:\Games. The library list should now show "D:\Games" as a Steam Library.
Exit Steam and then run it again. As soon as it opens, all the games that it finds in the library folder will be shown in White color as if they were already installed.
Method 2: Copying steam games from an old computer to a new computer On the new computer, open Steam and either install one small game, or create the subdirectory yourself - it should be C:\Program Files(x86)\Steam\steamapps\common
Exit Steam, then copy the games from the old computer's steam library usually from C:\Program Files(x86)\Steam\steamapps\common into the same folder on the new computer.
Run Steam, and all the game names listed in your library will be in White color as if they were installed. When you run the games, Steam will run the DirectX prerequisite installs if neeeded just like it had done on the old computer.
Method 3: Copying a single game from one PC to the other Steam can be left running. On the new computer, open Steam and either install one small game, or create the subdirectory yourself - it should be C:\Program Files(x86)\Steam\steamapps\common
Copy the game from the old PC onto the new one and place it in the same folder as before - usually C:\Program Files(x86)\Steam\steamapps\common\Arma 2 Operation Arrowhead
The game will still be listed in gray (not installed) in Steam. Click Install Game. Steam will displayed the message "Discovering existing files for xxx" and after a few seconds it will show the Play Game window.
Ubuntu linux snap
Copy the game directory to
/home/<yourusername>/snap/steam/common/.local/share/Steam/steamapps/common
For example:
~/snap/steam/common/.local/share/Steam/steamapps/common $ ll
total 20
drwxrwxr-x 4 john john 4096 Nov 9 11:58 'Proton - Experimental'
drwxrwxr-x 5 john john 4096 Nov 9 12:08 "Sid Meier's Civilization VI"
drwxrwxr-x 7 john john 4096 Nov 9 11:58 'Silent Hunter 3'
lrwxrwxrwx 1 john john 70 Nov 9 11:58 Steam.dll -> /home/john/snap/steam/common/.local/share/Steam/legacycompat/Steam.dll
drwxrwxr-x 5 john john 4096 Nov 9 11:58 SteamLinuxRuntime_sniper
To get steam to put the game in your library, you still need to click "install" and accept terms etc. If it detects the game files it should flicker "downloading" and skip straight to "validating" the files.
Your saved games and progress is perhaps lost. If not, each one will need personal attention.
I tried copying folders from ~/.local/share/<whatever>/ to ~/snap/steam/common/.local/share/<whatever>/ without any success. Others are invited to edit this answer if they work it out.
Because the user's home directory is accessible to snaps by default, it should not be necessary to issue any snap commands to give the steam process access to these directories.
