-50 has zero to do with drive errors OR prefs.

-50 occurs when there is corrupt or invalid media in your timeline.
Suggestions:
If you're using a long-GOP codec such as HDV, H.264, XDCAM, AVCHD or MPEG-2, I suggest you "optimize" to ProRes.
Make sure all images are RGB, not CMYK.
Ensure all images are less than 4000 pixels in width and height (just to be sure).
Get the "Pro Maintenance Tools" and use Corrupt Clip Finder to locate the corrupt clip.
After the corrupt file is located, use QT Repair or Media Salvage to fix it.
Make sure your files do not have Digital Rights Management (DRM) that prevent them being played outside of their intended application. Some files purchased from the iTunes Store will have DRM.
Make sure you are using AIFF or WAV audio files.
This can sometimes be caused by third-party codecs.
It can also be due to a corrupt render. Try trashing your render files and turning background rendering OFF and re-export.