Why this is, is because they come formatted as Windows NTFS drive which OSX can only read, so we need to reformat them so we can read and write – thats where a handy utility called Disk Utility comes to the rescue. The when the disk is selected in the finder bottom left symbol with the crossed out pencil means that the disk can only be read not written to. Initially external disks may be formatted for Windows and after you connect it to your Mac it appears in the device list in the Finder, but is a read only disk meaning that you can’t write to it in its current format. This will work in all modern versions of Apple Mac OSX including 10.9 Mavericks, 10.8, 10.7 and 10.6. In this tutorial we look at formatting disks via the GUI app called Disk Utility and its equivalent command line tool diskutil. All external USB disks can be formatted to work on Mac OSX, but not always straight out of the box.
