I have a Western Digital. I have a Western Digital external backup drive. I keep getting a message

Question

I have a Western Digital external backup drive. I keep getting a message that my last back up failed because my recovery drive (D) is full. I following the instructions (Desk Cleanup) to clean out old files but still get the same message and it tells me that I have a corrupted file. I don't know what to do and am rather computer illerate. Can someone help me?