Since you say you have a Mac, and not a PC, checkdisk is not a Mac program, but there has to be a Mac equivalent program in the Mac operating system. I have no idea what that may be. The disk likely has to be scanned for errors and fixed, so find the mac program and run it.
Check this google link for your best answer:
https://www.google.com/search?sourceid=navclient&aq=hts&oq=&ie=UTF-8&rlz=1T4GGHP_enUS429US429&q=Chkdsk+equivalent+for+the+Mac
Before you do this, try this solution, it just might help you, too.
