My WD Studio HD (500GB) used to back up to my MacBook Pro (Retina, 15-inch, Early 2013) but now when I plug it in, it won't show up in the finder or any of the WD apps. I'm using OSX Yosemite (10.10.4). Disk Utility shows it but under Verify, it says that it needs repair and when I try to repair it, it says it cannot be repaired. Disk Utility tells me to reformat the disk. When I try to partition it to delete everything and start over, DU tells me that the drive cannot be unmounted. I'm decent with computers but not great with DU or External Hard Drive things. Please help! I'd like to backup my laptop before upgrading to El Capitan.