Formatting an External Hard Drive for use with backups.  You have just bought an external hard disk to use for general data storage, Time Machine backups or SuperDuper clone backups. Most external hard disks you buy are pre-formatted for Windows. They will usually work with a Mac for general file storage, but even if your disk comes formatted for Mac, it may not work properly for backups. It’s easy to reformat with Disk Utility, and even divide up a hard disk into 2 or more volumes, but you need to do it before you use it because reformatting erases any existing data. 5/10