Get steam to realize I have games already installed

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.

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