I was reminded to Repair Permissions in Disk Utility. It still hung up after first disk of audio content for SoundTrack Pro, but this time it allowed me to select "cancel" install (which I couldn't do before). Then when I'm prompted to confirm the cancellation, I entered "no"... and now it's running fine.
There's only 50GB left on the start up drive - much less than usual. Sounds like I need to do some clean up. I read that Studio 2 SoundTrack files default to a different location than those in Studio (1) and may be duplicated. I'll need to check if this has happened and wasting space.