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.
https://community.wd.com/t/upload-stops-at-999mb/96492
https://community.wd.com/t/file-does-not-transfer-after-it-reaches-999-00-mb/96441
https://community.wd.com/t/wd-my-cloud-mac-only-uploads-512-kb-and-then-stops/96465/24
https://community.wd.com/t/support-os-x-el-capitan/96744
https://community.wd.com/t/my-el-capitan-wd-my-cloud-experience-so-far/96843
https://community.wd.com/t/mac-el-capitan-really-slow-connecting-finding-accessing-and-transferring/96814
https://community.wd.com/t/el-capitan-issues-solved-for-me/134988
https://community.wd.com/t/os-x-10-11-el-capitan-finder-connection-failure/96849
