Mojave on iMac preventing back ups

Hello, my iMac will not back up to my WD My passport Ultra since I upgraded the OS to Mojave. Has anyone had this issue and were they able to resolve it? Apple Support have suggested WD will need to bring our a firmware update as they did for High Sierra but I never had an issue with that.

Hi Lynad001,

WD drives are already updated and it seems like it is the system specific issue so just restart time machine and restart you mac also and then try to create time machine backup after that.

Hello, thank you for getting back to me. I spoke to WD support and they said the same thing so I went back to Apple and they are currently investigating as it appears to be something with Time machine itself.