EX2 USB back up fails

My Cloud OS 3 Personal & Network Attached Storage My Cloud EX Series
1

Thanks in advance for any help as i am no tech !!

I have a WD My cloud ex2 on a mac, when trying to do a usb back up it runs for a while and gets a lot copied then it says its filed i get it to carry on then it may or may not fail straight away.
am i doing something wrong or is it a fault?
thanks

2

Is this happening with any external hard drive?