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You can lock a track to prevent accidental changes. While a track is locked, you can’t record on it, or edit or create new regions. To edit the track, unlock it, make your changes, then lock it again.
In GarageBand on Mac, click the Lock button 
in the track header.
If the Lock button isn’t visible in the track header, choose Track > Track Header > Show Lock Button.
The lock turns green to indicate that the track is locked.
To unprotect the track, click the Lock button 
again.
In GarageBand on Mac, click and hold a Lock button , then drag the pointer up or down.
The Lock buttons of all swiped tracks switch to the same state.