I've been happily playing around with GarageBand for a while now, and I love it as a simple musical sketchpad. I've just upgraded to a nice new MacBook, and I've been getting a bit more adventurous with my recordings, but I'm running into some issues with the dreaded "GarageBand cannot play back the whole track" errors. I've tried storing the stuff on an external 7200rpm HD (faster than the internal 5400rpm drive) and I'm still getting errors... has anyone got any tips on how to overcome this? The song I'm working on still needs at least two more tracks! 
