I get a ‘Failed to write MBR’ error from the disk writer on macOS.

This is a known bug in DCP-o-matic, which seems especially to affect Monterey and Ventura. These steps should work around it:

• Go to ‘System Preferences’
• Go to ‘Security & Privacy’
• Go to the ‘Privacy’ tab
• Scroll down to select ‘Full disk access’
• Click the lock and enter your password
• Click the + and navigate to /Library/Application Support/com.dcpomatic/dcpomatic2_disk_writer