My name is Rohit and I work for the Social Media and Community Team at Dell.
I regret to hear that you are facing an issue with your dell system. I want to update you that back-up happens in D Drive on your system and you need to empty that occasionaly to ensure that you have enough space for next back-up and if D Drive don't have that much space it will give that error message and will not be able to save the back-up. You can try that option.
Waiting for you replay.
Glad to assist you!
Thanks and regards
Dell Social Media Responder