Peter,
I've heard that older versions of Garageband have had limitations on how long you can record based on the amount of music measures. The most you can have was about 999. People have been able to get around this by adjusting the projects BPM and measures. I would try setting your BPM to something like 40 and see if that changes anything. Give that a shot and let me know if anything changes.
Cody Walters
JW Studio LLC
Houston Video Production
Houston Wedding Videos
Final Cut Studio 3
Adobe CS5 Master Suite
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