A short, but accurate description of your issue.

My pc wants to re-download Black Ops 3 even though it is installed.

What is actually happening?

Steam starts a 80 Gb download. When i look into steamapps, all files are there.

When did it start happening?

Black Ops 3 crashed once and suddenly none of my workshop mods/maps were listed in the game anymore. Restarting the game, steam and my pc made no difference. All workshop-files are still on my pc ("C:\Steam\steamapps\workshop\content\311210") but it still wanted to start a +100 Gb download for "Black Ops 3 - Workshop Content"

Take screenshots

Here is a video of the problem: https://youtu.be/6g2zu6MKjGE Please read the fixes i already have tried.

What OS/Platform are you using?

Desktop, Windows 10 Pro x64 1809

Information on what fixes you have tried. If you haven't tried anything you should state that in your comment.

I've tried:

• removing the *.acf files.
• renaming the BO3 folder to "Call of Duty Black Ops III123" so Steam can't find the folder anymore, uninstall the game in steam, exit steam, rename folder back to "Call of Duty Black Ops III", launch steam, install game to the drive where the files are located, even after discovering the existing files it still wants to redownload the files (see the video i have linked above)
• some youtube tutorials from people who have the same problem but nothing helped

Speculation on what might be causing the issue.

I have no idea