There are several threads found with the forum search feature (magnifying glass icon) on this particular issue and how El Capitan in general has problems with the WD My Cloud.
http://community.wd.com/t/upload-stops-at-999mb/96492
http://community.wd.com/t/file-does-not-transfer-after-it-reaches-999-00-mb/96441
http://community.wd.com/t/wd-my-cloud-mac-only-uploads-512-kb-and-then-stops/96465/24
http://community.wd.com/t/support-os-x-el-capitan/96744
http://community.wd.com/t/my-el-capitan-wd-my-cloud-experience-so-far/96843
http://community.wd.com/t/mac-el-capitan-really-slow-connecting-finding-accessing-and-transferring/96814
http://community.wd.com/t/el-capitan-issues-solved-for-me/134988
http://community.wd.com/t/os-x-10-11-el-capitan-finder-connection-failure/96849
