"Preparing to copy" takes exceedingly long (Mac)

Others are reporting issues with El Capitan with the My Cloud. Do you have another computer (non El Capitan) that you can test with to see if the copy problem is confined to just that one Mac?

http://wdstagingsplit.staged-by-discourse.com/t/support-os-x-el-capitan/96744

Also take a look at the following thread which may provide some additional help.

http://wdstagingsplit.staged-by-discourse.com/t/before-you-pack-up-your-wd-and-return-it-lets-talk-about-copying-speeds/91887