Error Code 13

I had to cancel the first backup of my Nexus 7 because, after an hour, I had over 18,000 errors similar to:

Scan failed (-1): Scan failed on [/acct/uid/10000/] with error code 13

or

Scan failed (-1): Scan failed on/proc/12153/task/12220/cwd/proc/12153/task/12220/cwd/proc/12153/task/12159/fd/] with error code 13


The backup was to a new folder on my PC running Win 8.1 64 bit

Some help would be appreciated.

Thank you