and if you have an external HDD and wish to start an iTunes Movie Library like I do
(I keep music on Internal)
Quit iTunes, Re-open it while holding "Option" This will prompt to use existing Library, or create new one. If you create a new one on the external, you'll be brought to that empty library to start adding movies.
.wmv
.avi
.flv
these are some that don't work in iTunes -- to make it work, Convert them using third party apps like iSquint.
If you wish to return to music library, quit iTunes, re-open (while holding "option" )
This time select an existing library file >home folder>music>iTunes
I made alias's for my two libraries, and placed them in one folder for easy access.