Blu Ray error when trying to burn disc...

Everytime I attempt to burn a Blu Ray dvd from the share function in FCP X or in FCP 7 I get this error: please check if your boot volume has sufficient space.... My boot drive has over 100 GBs left in space, I don't know if that is the same as a boot volume, but why can't ZI ever make a damn BR dvd?

Now a window comes up saying: please insert media to burn the disc...