I realised another aspect to this. My last post on the matter primarily related to adding new content. What if you’ve got content that you want to split? Well, iTunes is very clever at tracking changes in file locations, at least Mac OS X is. I’m not sure if this is as good in Windows.
If you’ve got iTunes open and you go into your music folder in the Finder and move physical files to another location, iTunes notes where you put them. For example, you could move the Movies folder to another drive and iTunes would track the change. Use this technique at your own risk, however, because I’ve seen situations where iTunes didn’t track the change. I was in a hurry so I didn’t debug this at the time. I’ve even seen iTunes find a file’s new location even though it was moved when iTunes was closed, although I haven’t really tested this.
Sorry it’s a bit theoretical but I don’t manage my library this way and I’m unwilling to run tests of this nature on it “to make sure”, because I don’t want to mess it up. These observations, however, could be put to use by users with the need to split their library.
In the long run, my advice is to get bigger drives, either internal or external. Hard drives are astonishingly cheap these days.