Switching iTunes Library to External HD

If I understand correctly. You can start iTunes up while holding down the option key. This will allow you to choose your library or create a new one.

As Bob mentioned, here are the specific instructions...
How to open an alternate iTunes Library file or create a new one
http://docs.info.apple.com/article.html?artnum=304447

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).
Patrick

By the way and for what its worth.... Apple iPhoto allows thge use of multiple libraries as well. To access them from iPhoto you hold down some key or another, (not sure which) when starting up and then select the desired library. That becomes the default library for photos. There is a good shareware utility called iPhoto Library manager that makes this easier and gives you menus for it. There is most likely something just like iPhoto library manager, only for iTunes since its the same basic idea.

emhjay,

I think I understand what you are trying to do, as I have done it myself.

First, I have 3 iTunes libraries. I use the shareware software "Libra" to manage them. One is my everyday library that I keep ON my Macbook Pro. One is my 'Master" library that I keep on an external hard drive. The 3rd is my new "iphone" library where I have ripped everything to a lower bitrate so I can fit more on the phone. Libra allows me to jump between the 3 as I need to, keeping seperate playlists and locations for each set of files.

On my everyday library, I keep all my music on the Macbook, but the videos stay on an external drive. Once the videos are on the computer, there are two ways I manage the file and its location.

For videos I download from iTunes: It downloads to my MCP HD, and once a week I move them to the external drive. I have folders set up for TV/Movies/Music Videos. I used the Advanced preference to unselect "Copy files to itunes library." so itunes knows where the file is and that it's outside the regular library. When the external drive is not connected, I get the exclamation mark, but they remain put in the playlists. You cannot do a consolidate library after doing this.

The other option would be to move the default location of the library by using the Advanced Preference pane. Then download the podcasts and move it back if/when you add music/videos, but otherwise it can stay put. THis might be a pain however depending on how often you add non-podcast content.

There's likely other options as well, but that's what I use. I like Libra a lot, by the way. I use it regularly.