Based on your comment about using the copy command in cpanel I would suggest you are probably running out of diskspace.
You mentioned you do have ssh access available and I an unfamiliar with cpanel so if you can ssh into the server you should check your diskspace with the following command:
There is also a possibility that you are running out of inodes although unlikely since it is a single tar file.
If you have ssh access you will be much better off moving the file instead of trying to copy it.
mv /home/username/cpmbackup.tar.gz /home/username/public_html/cpmbackup.tar.gz
This will happen almost instantaneously instead of waiting and hour or more for it to copy. It also will not use up any additional disk space.