Time Machine backup failing

I Have an iMac 27" currently running Yosemite. I took my Mac into the Apple Store to have the optical drive replaced. While there they ran diagnostics and ran a program to get rid of adware and help speed up the machine. I was encouraged to upgrade to El Capitan. In preparation for that upgrade I am trying to run a manual backup to preserve the deletion of all adware from the system. But my time machine will not work. I have gotten a backup failed message. I reboot the system after unplugging the WD MyCloud from power and from the Mac. I plugged them back into the Mac and am trying to run a manual backup. Now it has been stuck in "preparing backup" mode. Any suggestions??