GarageBand lets you shorten Real Instrument loops by moving the start and/or end point. This works for the end point with Software Instruments, but not the start point. It seems immovable. I found a simple way to beat this using another feature of GarageBand for cutting loops:
- Place the loop (whose start position you want to change) on the timeline (let's call it LoopA).
- Listen to and identify where you want to start LoopA from.
- Get any other Software Instrument loop (let's call it LoopB); drag and drop LoopB onto the LoopA's track on the timeline, away from LoopA. You need this step so GarageBand will show you LoopB's endpoint in the next step.
- Now drag and drop LoopB onto LoopA so that the endpoint of LoopB matches the new start point you want for LoopA.
- Discard LoopB and any other remnants of LoopA.
[
robg adds: I haven't tested this one...]