Garageband Drum Question - half measures
Here's what I did


Use an empty 1/2 measure to make the drum fill sound right (even though it's in the wrong place)
Create a midi track just below your drum track, and set the midi instrument to the same one that the drum is using
Copy/paste the drum track into the midi track and now mute the drum track (note: opt-drag doesn't work here, has to be copy/paste)
Now edit the midi track as needed: drag left to close the empty 1/2 measure in the midi track