Multiple Music Libraries in itunes

I keep some of my music on an external disk, however when the disk is not plugged, itunes cannot find the files and stops playback. One fix for this is to have multiple itunes libraries : one for both the files in the internal and external disks; and another for files only in the internal disk. To create and select which library itunes will use, hold down Option in Mac (Shift in Windows) when opening itunes. It will ask you which library to use; and whether you want to create another library. Typically, your current library resides in the "itunes" folder in your "Music" directory; another directory will be created inside the "Music" directory. I have named my libraries "ExternalInternal" and "Internal"; and when it asks you which library to use, typing the first few letters will take you to the matching folder; so that you don't have to scroll down to find them. There's also an app called Libra which takes care of this issue, though I haven't tried it yet. I assume it was useful when this feature was not implemented yet in itunes, i.e. pre itunes 7.