If you have been using your WD Elements on both Windows and Mac, it is highly possible that the drive is preformatted in NTFS. If so, you may need to change the drive’s format in order to use it on a Mac PC. If you plan to use the drive on both systems, I would recommend you to format it in exFAT which is a file format supported by both Windows and Mac.
Right now the file system may be supported only by Windows and that is why you see the files only on Windows PCs. The other hard drive may be formatted in exFat and that is why the files are visible on both systems.
Please backup all data before formatting your external hard drive. Let me know if there is any progress.