The 999MB upload issue while in local network and the 512KB upload issue while in remote happens when you are using the My Cloud Desktop app in a Yosemite 10.10.5 or El Capitan. Something happened with the last Yosemite update as in 10.10.4 this works fine.
For the moment the only workaround, as stated above, is to map the drive when you are in local network or using the web mycloud.com when you want to transfer remotely.
The issue is not yet resolved and don’t know whether Apple, WD or both have something to do to fix it.
