Backup is done on a WD My Cloud ...
Unfortunately, that is not a supported Time Machine configuration. To learn how to use Time Machine read Use Time Machine to back up or restore your Mac.
Despite their manufacturers' insistence to the contrary, there are no non-Apple NAS devices that will reliably work with Time Machine. If you want to use your NAS device to back up your Mac's contents, then use a backup strategy other than Time Machine.
The exhaustive list of devices supported by Time Machine are as follows:
- External drives directly connected to the USB, FireWire, or Thunderbolt ports on your Mac
- The built-in drive of an AirPort Time Capsule
- A USB drive connected to the USB port of a Time Capsule or AirPort Extreme 802.11AC base station
- A networked volume served by OS X Server using Apple File Protocol (AFP)
That is all.
Use whatever backup device you want, but you should be aware that this site is full of reports of misery from hapless individuals who had been using third party NAS devices for Time Machine backups, only to find that they were incomplete, corrupted, or useless in the dire circumstances in which they were required. Apple won't care if you lose your data while using a Time Machine configuration specifically excluded from their technical support documents.