Is there a possibility that one of the files you are trying to back-up is in use at the time? For example, I use a program called Studio to edit my videos; if I try to copy project files to another disc whilst the program is live on my computer the copy fails. If you are trying to copy the whole of your C drive to another drive, that will fail because some of the files are in use by the operating system, so are protected.
Try copying your files to your back-up drive a directory at a time and see where it fails.