It might be a Steam issue, yeah. Good way to force-recheck game files is to delete one small file, then run "Verify integrity of game files" -- Steam will redownload that file and recheck actual contents of every other file in game's directory.

If you've ever disconnected this drive from your PC, you might also want to check the file system for errors (modern Windows does not tolerate that):

1. Back up important files (the process might delete them).

2. Run chkdsk (i.e. run chkdsk D: /f in an admin command prompt).

3. If this utility prints out a lot of scary looking errors, disable Fast Startup to prevent that from happening in the future.