My book for Mac

I am new to Mac and struggling with back up to WD external drive “My book for Mac”

I set up my iMac to automatically back up that happened for a while but has not done so recently.
Am I missing something
Any help will be greatly appreciated

Hello stevefarrington,

Are you able to transfer data manually over the drive?

If you are using Timemachine to backup, have you checked the time machine drive preferences?

Please refer to the below link for Online User Guide and Solutions of Timemachine:

https://support.wdc.com/knowledgebase/answer.aspx?ID=19531