How to change the primary storage drive?

Solution 1:

Yes and no. You can move your profile over to the external drive to make that your profile home by going into file management and move your profile. This will take care of profile and all save data going forward. You'll have to manually migrate all other game specific save data over and may still get prompted within games on what device to save to.

As for game installs. Those will always default to the internal HDD. You should then be able to move the game, after it is installed, to the external drive. The only exception to this is the 4GB XBox 360 model which will prompt you on where to perform the install.