How to open an alternate iTunes Library file or create a new one
So what you can do is open/start a brand new library on your external drive. The use the File -> Add to Library... command to select and add all the old stuff on your internal drive. It will copy them to the external and add them to the library. This would be your main library.
Now when you need to travel and do not have the external you can use the same instructions above to start iTunes and select the internal library and then use just that.
If you want to change the internal library, while the external is hooked up, you can delete stuff out of in the internal library or add things from the external folders you want to transport with you (which should copy into the internal drive when added if you use the iTunes default setting to copy added stuff to your iTunes library).
With version 7, using the technique in the link above, when you create a new library or open an alternative library, the library is all self contained with the database files and whatnot in the same location. So when you open the library on the external, anything you do will be done on the external including adding new stuff. When you switch back to the internal library, then everything you do will be done with that library.
Message was edited by: PT