Like xpcman said, you may be facing some file format compatibility issues. It is a common problem when using these two operational systems. Please connect the drive to the Windows PC and open Disk Management. Does the drive appear and does it have a file format?
What you should do is to backup all data on the Mac PC and then format the drive choosing the exFAT file system. It is compatible with Windows and Mac and you should be able to transfer files between PCs.
Hope this helps