Thanks. No backups; this Mac Pro was purchased as a spare for a working studio and was just put into service. It had the graphic card problem covered by Apple's recall of 6-core Mac Pros, which was causing it to freeze. I took it into the Apple Store and they repaired it but they installed 10.12.3, on which Pro Tools is not validated. Our main Mac Pro (12-core) is running 10.11.6. We wanted to take the opportunity to set up the spare with ProTools on Sierra. Perhaps we're stuck on 10.11.6. I don't understand why Apple can't make prior installers available. :-/