Ran into an issue this weekend, so I'm memorializing it here.
I pulled the drive this weekend to put into another machine I'm using for temporary file storage while I migrate Synology servers (longer story). This 1TB drive was holding my Steam Library.
Fast forward a week and I have picked up a used NVidia card, and wanted to test it out. Went to download/reinstall the game and Steam says there's "0 bytes available" even though there's no other Steam folder, and the DEFAULT is set to the C:\ drive.
Here's what I did, that wasn't covered by other posts and threads I found:
1) Go to Steam Settings
2) then Downloads
3) then click Steam Library Folders
4) then right-click on the directory shown and click on Make Default Folder. (yes, yes, even though it says it's already the default)
5) then right-click on the directory again and click on Repair Library Folder. Steam will restart.
You're done. Try installing games again.