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In the Finder, click on "Go" in the menubar and choose "Utilities"
Locate the "Disk Utility" app and open it
In the left hand sidebar, choose your external WD Hard Drive
then click the Erase tab.
From the Format pop-up menu, select "Mac OS Extended (Journaled)", type a name for your disk, and then click Erase.
After the disk has been erased, choose Disk Utility > Quit Disk Utility in the menu bar at the top of the display.
It should now be ready for use on your Mac.