External hard drive shows partly full although there are no files on it

I want to reformat an external hard drive and for this purpose I saved all files I had on it onto my internal hard drive.

But although I deleted all files from the hard drive, in the explorer it still shows around 17gb of the ~100gb drive are full. I have activated showing hidden folders, but it's still empty.

Now this doesn't stop me from formatting the drive, but I don't want to accidentally delete something that I just don't see.

Why is the drive not empty and is this normal? Can I safely format it?

Answer

You've selected Show hidden files, folders, and drives, but be sure to also deselect Hide protected operating system files (in Folder Options > View). Otherwise, you're getting an incomplete view.

(Cf. How to delete $RECYCLE.BIN folder on external hard disk?)