Formatting an External Device

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When switching from a PC to a Mac you may find that you are unable to write files to an external hard drive you had been using with your PC. If this is the case this is because your drive is formatted with NTFS, a file system that you can only write files to with Microsoft Windows. If you want to write files to it from your Mac you will need to reformat it.

WARNING: Formatting your drive will erase all the files currently on it. Back up all of the files on the drive before erasing it.

  • First you need to find and open the application Disk Utility. You can find Disk Utility in /Applications/Utilities.
  • Now, find your USB drive in the list on the left hand side, and click on it, the one you want to choose is the one with its icon on the far left hand side of the colum, rather than the indented one as you can see below.


  • Now, click on the 'Erase' tab.


  • Opening the 'Volume Format' drop-down menu, you can see that we have quite a few options.


  • From the drop down list shown above there are two sensible options to choose to format your disk.
    • Choosing 'Mac OS Extended (Journaled)' (also known as HFS+) will make your drive compatible only with a Mac, and a Windows computer will not even be able to see what is on it. (unless MacDrive or another similar program is installed on the PC.)
    • Choosing 'MS-DOS File System' (also known as FAT32) will make your drive completely compatible with both a Mac and Windows computer, but you cannot make a file any larger than 4GB. This is the same as formatting as FAT32 under Windows, however unlike Windows there isn't an artificial restriction which limits creating disks in FAT32 format which are bigger than 32GB.
  • Whichever option you choose, you are now ready to format your drive. If you wish you can choose a new name for your hard drive, after doing that click the 'Erase' button and you will see a warning, press the 'Erase' button on the warning to continue. You will now have to wait a short time while your drive is formatted, once this is done it will appear in the Finder and you can now write files to your external hard drive from your Mac.