About file sharing between mac and pc

i bought a mac mini for my home server for file share for two external HDD (and airvideo but i know it works) with movies for the family i setup up the file share in the sharing tab in system perferences with one external HDD to have a test to see if it works but i can access the mac mini computer on windows and I can't access the external HDD it comes up with this error wen i am trying to access it heres a screenshot of the error.

if anyone can help it will greatly appreticated.