Hello! I have an external hard drive to which I've previously been able to use with both my Windows 7 PC and my Macbook. However, although the PC can still access the drive, it simply stopped working with my mac and "cannot be repaired." I could probably re-format the drive (A WD My Passport), but I want to know why so I can prevent this in the future and hopefully fix the issue without reformatting the drive. Thank you!