When I try to burn a Blu-Ray disk in FCP7 via the command 'Share', after a few seconds I get the message: "Apple Q-Master file agent not found' and the burn is aborted. It worked perfectly before, so I'm puzzled that is does not any more. What goes wrong? OS is Snow Leopard on a Mac Pro.

This is a common problem, at least for me on my old MacPro. This would crop up if I had Compressor set-up for multi-processing. First quit all the program. The solution is to go to your Library folder, into Application Support and throw out the Apple Q-Master folder. Restart Compressor, this will rebuild the Qmaster folder. You should be go to now.