People are having various issues with El Capitan and the My Cloud. Only suggestion I have is to install the WD My Cloud Desktop for Macintosh software (http://download.wdc.com/nas/WDMyCloud_mac.dmg) on your Mac and try it out and see if it works. The software is generally supported on Mac OS X 10.6 and up.
See the huge El Capitan thread below or do a forum search for El Capitan to see the various issues others are having with that OS.
