A lot of problems people have with mythtv can be traced back to filesystem permission issues.
Unable to record, or watch live TV
You have set wide-open permissions but still can't write to a new disk you just installed:
$ ls -l /media/data/mythtv/ drwxrwxrwx 2 mythtv mythtv 4096 Jun 11 12:08 recordings
On Linux, and possibly other UNIXes, check the permissions of the underlying mount point:
$ grep mythtv /etc/fstab /dev/sdb1 /media/data ext4 defaults 0 2 $ sudo umount /media/data $ ls -ld /media/data drwxr-xr-x 1 root root 4096 Jun 10 11:08 /media/data
Also: check the permissions of each step in the path from /.
Reference: Thread on mythtv-user titled "Can't watch TV Permission Denied opening recording file despite chmod a+rwx"