Scinx,
I think I know what you are trying to do, unfortunately you can't do that using the edit functions. As far as I know cropping is not available on the HD24, you need to copy/paste what you want to keep into a new song.
Using "cut" will just record silence between your edit points.
All your tracks are kept the same size, and the longest track you ever recorded in a song will dictate the size of all tracks. In other words, the recorder doesn't keep track of (and dynamically manage) individual track lengths. I think this is by design to make it simpler to manage the cut/copy/paste/undo fucntions.
For that matter I don't think you can reduce the track count in a song either, you would need to copy your tracks into a new song with less tracks.
There is a clue on top of page 33, "A Song of a certain length takes up space even for tracks that were never recorded on".
Well, this is my understanding of the HD24, and I could be totally wrong of course.