Mac Geekery

Answer

does that make any sense?

After about six reads, yes.

So you would like to have all of your music and your iTunes library file on an external drive and when you’re near that, use it. When away from that drive, you’d like a subset of music on your computer to listen to. Additionally, you want to be able to add music to your iPod from either library.

iPod question first: use manual mode. You can then drag music into the iPod from any iTunes library on any computer.

iTunes can be set to use another library rather easily. Hold down the Option key as you start it and it will prompt you to make a new library or use an existing one in another location. Create a new one on the external drive. When you do this, it will make a folder and put the library file inside. Drag an alias of that somewhere useful, and one for your mobile library as well, and then start iTunes with the library files.

There’s a catch, because there’s always a catch: iTunes’ preferences for where the library file is, where the music itself is stored, and also how to organize it is all stored in iTunes’ plist preference file, for which it does not have built-in switching. Software to the rescue, of course. Get rooSwitch and setup a profile for iTunes. Then use rooSwitch to start iTunes in a certain profile when you want to use it. This has the added benefit of remembering which library you wanted to use depending on which preference file is in effect.