My guess is that since you are using an Elura50 that you may have had it set to some “a-typical” settings when the footage was shot. FCPX must have the codec for your settings cause it plays back, but it sounds like quicktime doesn’t… have you performed a software update for quicktime recently? If you can find the proapps QuickTime codecs update, that might allow for quicktime playback.
Another option, since you may have shot your footage with settings that might trip up many systems… you could export quicktime movies of your footage from fcpx. That way, moving forward, your archived footage should be fine for playback and importing to future NLE’s.
Sorry I couldn’t be or more help! Good luck.
-Fred Waycoot
Editor/Colorist/Filmmaking Guy