You can also move the apple loops to your external drive. The folder is:
HD> Library> Audio> Apple Loops
Then create an alias of the folders new location on said ext. drive and move the alias to above folder so when Logic looks for the apple loops it will be redirected to the new location.
However, I've never tried this with apple loops and have no desire or need to do so. I have done this with exs instruements and other samples and third party instruments .dat files with great success.
If for some reason Logic can't reference the moved apple loops move them back.
Another way of doing this is to install logic 8, deselect the extra content you don't want on your internal drive. Select where you'd like the extra content to be installed to as Christian suggests.