Several months ago I downloaded a trial of Lightroom as I sought to find an alternative to Aperture. At that time I didn't really pay attention to what I was doing when I was at the library setup stage. So, I currently have some pics in LR (imported from Aperture) and other LR-only pics. Anyway, here's a couple of questions:
 is there a way I can start from scratch in setting up Lightroom, so I can go through the formal Aperture=>LR migration?
 before doing the migration, I assume it is best to spend some housekeeping time with Aperture to clean things up as much as possible.
 In my Aperture library, I have about half my photos on my computer drive and the other half referenced to an external drive. In this sort of migration, how to I mange this?