Make sure your external HD is mounted.
Start iTunes with the Option key held down. (That's the one with the symbol that looks like a shopping cart without wheels, also called "alt" on some keyboards.)
You'll get a dialog asking you to choose a library.
Start iTunes with the Option key held down. (That's the one with the symbol that looks like a shopping cart without wheels, also called "alt" on some keyboards.)
You'll get a dialog asking you to choose a library.