Hi all I have an external HD I use to back up my music files and it's worked well but since I upgraded my OS to Sierra recently I can no longer add anymore files - my permissions are 'read only' and cannot be changed. I checked the format of the drive and surprisingly its Windows NFTS! I guess I'll have to reformat it and load my files on again, but how has this happened? How was I using it before if it wasn't formatted for mac? Cheers.