Did you remove the external USB drive after making the initial Safepoint backup?
If so some have reported problems with the My Cloud not properly recognizing the reinserted external USB drive. In many cases no Safepoint is found on the drive which may cause Safepoint to start a new backup of the My Cloud. The cause for that particular issue is due to the My Cloud changing the name of the USB Drive share when it is attached to the My Cloud. More information on that issue can be found in the following threads, including ways to work around the issue.
https://community.wd.com/t/new-release-firmware-release-04-04-02-105-1-27-2016/148441/69
https://community.wd.com/t/safepoint-not-visible-after-v04-04-02-105-firmware-upgrade/150993
https://community.wd.com/t/missing-safepoint-after-firmware-update-ver04-04-02-105-2016-jan/149031
https://community.wd.com/t/re-establishing-lost-safepoints/148609
