[David Sarmiento]

THE ANSWER!!!!
FINALLY!

HEY, i was having the same problem just a few minutes ago. i was looking here for some solution…
well, I finally found that the problem is to match the aspect ratio at the sequence settings with the aspect ratio used by your device/deck/camera.
i had a video shot on xdcam ex… exported to quicktime in apple prores codec.. then i started another FCP proyect with DV-NTSC firewire configuration (easy setup)…
you have to try diferent kind of aspect ratios in the sequence settings window… until you can playback in real time and watch the video in your firewire device.
I put aspect ratio: NTSC-DV (4:3)..
pixel ratio:square… and checked anapmorphic
that way it worked fine.

i hope this can help you guys!