Backing Up Songs on PC Problem

I am using sync back for backing up my data. I encountered this problem when I was backing up some music on my PC.
I got an error that said error 'Failed to copy source: Cannot move file (5): Access is denied'.

Error:
7-Zip:Internal error,code 105.
While some of the files were backed up perfectly, it refused to back up the rest. I am not sure what the difference is between the folders it allowed to back up and the ones it refused. If you have any idea about this sort of things, please share your thoughts with me.
Thanks for your interest.
