Use the Mac's built-in Disk Utility app to prepare any external hard drive or flash drive to be used with your Mac.
If you plan to use the drive for Time Machine backups, this article will outline all of the steps required to do so.
Most external and ready-to-use hard drives are shipped with PC/Windows formatting. Although Macs are able to open and read files on Windows-formatted disks, the functionality is limited, and in certain cases like Time Machine, a PC-formatted disk can't be used at all.
Completing the steps in the first section of this article will allow you to confirm that your Mac recognizes the drive, and upon being plugged in, displays an icon for that drive on the desktop. This is accomplished by first setting some initial preferences in Finder.
1. Unplug and eject any external or flash drives that may be plugged into your computer
2. Shut down then boot up your computer and log into your computer's user account
3. Click on the Finder icon on the program dock
4. Click "Preferences" from the Finder menu
5. Ensure that the "Hard disks" and "External disks" items are ticked, then close that window
WARNING: Successful completion of the following steps will result in permanent data loss from the drive you are formatting. Exercise caution with this process. Remember to unplug any other drives you may be using (in step 1 above) from the computer prior to beginning this process. Only plug in the drive you wish to completely erase and format for Mac.
1. Click the "Go" menu, then Applications, double-click Utilities folder, double-click Disk Utility
2. Plug in the external drive. It should show up on your desktop and also in the Disk Utility window. In our example, the drive is the Crucial Gizmo! JR. Media, with one partition named "UNTITLED", which is 8GB in size.
Conditional: Choose "Don't Use" if prompted to use that drive with Time Machine backups. This will only occur if the drive you plug into the computer has more available free space than the internal hard drive itself.
3. Click on the partition name and click the "Unmount" button
4. When the volume is unmounted it will appear dim as shown here:
5. Click on the drive -- in our example, the drive is the Crucial Gizmo! JR. Media -- and click the "Erase" button
6. The dialog below should appear. Choose "Mac OS Extended (Journaled)" as the "Format".
7. Input a name for your drive -- avoid using spaces, special characters, or long names. Choose "GUID Partition Map" and click "Erase".
8. Click "Done" in the dialog box once the process has completed. If you receive the message related to using that external drive for TM backups, choose "No". (You'll set up the drive for Time Machine as part of a different process, which we outline here.)