Hmm, that's quite a chunk of space taken up. Fortunately, the issue need not indicate the card itself is definitely bad, but could be the result of many things relating to file size reporting, incomplete file deletion, bad formatting, (formatted in a different machine?), corrupt files, etc.
Assuming that you don't want these "phantom" files, I'd suggest that you just pop the card into your recording box of choice & format it there. The recorder should then report available space (which you can compare with a newly formatted similar card if you have one). Hopefully, if you then check the card in your p/c, it should show full capacity - less the usual nominal amount for internal workspace, etc. If you still get the reported error, probably best to chuck the card as it's not worth risking your recordings with a card which is even slightly questionable (IMHO!)