I also want a way to clear this. And I've tried uninstalling every game, removing every library, and then deleting every library folder and every instance of "libraryfolder.vdf"

Steam still has a record somewhere of installs that happened on drives that don't exist anymore, giving this error. Worse, it is messing with other apps that access my Steam library. They get sent searching for entries that don't exist and I think one that is bugged and causing more problems.

Is there any way to fully remove all past library information? Or must I uninstall Steam, and then hunt down every file left behind to clear these ghost libraries.

Currently info is held here .../Steam/steamapps/libraryfolders.vdf and a duplicate stored here .../Steam/config/libraryfolders.vdf

(Things are pretty straight-forward but just in case someone unsure of things comes across this post, make a copy of .../Steam/steamapps/libraryfolders.vdf just in case you want to revert stuff)

You could remove the file at both locations and end up with just the Steam installation Library, all other Libraries removed.

Or you could open .../Steam/steamapps/libraryfolders.vdf with Notepad/text editor and delete the entry for specific Libraries (or even just specific installed apps from a Library).

After removing the files/saving changes made to the file, restart Steam if it had been running.

Deleting them did not stop steam referencing drives that don't exist. I'll try editing

Hmm, I did not expect that.
For me, removing the files then starting/restarting Steam results in the files being rebuilt with just the Steam installation Library... ¯\_(ツ)_/¯

Is the missing drive shown in the Storage settings? If so, can you simply remove that Library drive?