A better way to write files as root is to pipe the output into `tee`. eg:

   blah blah blah | sudo tee -a file

This will do the appending as per your example. If you want to write to the file like people are doing with the Ubuntu image then just drop the append (-a) flag:

   cat ubuntu-14.04-desktop-amd64.dmg | sudo tee /dev/sda1

Though obviously the `dd` utility is still a better way of writing disk images than any of the above.