You may be facing a file format compatibility issue. If you have used the hard drive on Mac, it may have been formatted for Mac usage. In that case it may not work when connected to a Windows PC. You have backup of all the files on the drive, right?
In that case, connect the drive to the Windows PC and format the drive in exFAT file format. It is supported by both systems and you should be able to transfer files.
Hope this helps