I believe that this was caused because the full command needs to be run with elevated permissions.
The following will not work:
:~/.ssh$ sudo cat ./id_rsa.pub >> /tmp/nfs/root/.ssh/authorized_keys
Where as the following will work:
:~/.ssh$ sudo bash -c "cat ./id_rsa.pub >> /tmp/nfs/root/.ssh/authorized_keys"
or this will work:
:~/.ssh$ sudo bash
:~/.ssh# cat ./id_rsa.pub >> /tmp/nfs/root/.ssh/authorized_keys