If it were me, I’d be tempted to move the data off somewhere safe and then just reformat the drive, then put the data back onto it.
There are software that you can install on your PC which will show drive usage broken down by files/folders which you may also want to try. It will probably be something system-related like a swap file or the recycle bin file that I mentioned before, which has been dumped/created on the drive by mistake and is now orphaned but taking up space. They can always be a “fun” challenge to track down, a drive analyser program can help there quite a bit.
Personally I haven’t needed one for a while, so I’m not sure any recommendation would be up to date. But from a quick Google search there are a few options: