WD 4 TB Elements Desktop External Hard Drive - USB 3.0

Hi, I have a WD 4 TB Elements Desktop External Hard Drive - USB 3.0. I copied files across to it OK. But suddlenly when I want to transfer more it says "Remoce the write protect or use another disk. There is plenty of room on this disk so why is it suddenly asking for this? Anypne got a step by step advice?
Thanks a Silver surfer