by Visvanath Ratnaweera.
During mounting (the mount command or /etc/fstab) you have to set parameter for it to be writable by a user. You can always test by doing something like (example Debian/Ubuntu):
$ sudo -u www-data touch /path/to/backup/xxx
$ sudo -u www-data mkdir /path/to/backup/yyy
$ sudo -u www-data touch /path/to/backup/xxx
$ sudo -u www-data mkdir /path/to/backup/yyy