We need to know what are you using to make the backups? Time Machine requires that the external backup drive be formatted to HFS+. CCC and SuperDuper likewise require the backup drive be formatted to HFS+. Unless you're using some other backup program, you would need to maintain the HFS+ format. You can, however, mount a HFS+ drive and read it from a Windows computer with the proper driver installed. Again Paragon software would be my choice for the driver software.