You should be able to edit in FCP5 if captured w/ FCP6, so long as you’re using the same version of Quicktime, but you’ll need to start a new project and add the clips manually. Final cut projects are not compatible with previous versions, even if it’s only a point upgrade.
if you’re using those same settings for the SD video you upload, you’re giving the SD video roughly 4 times more bitrate than the HD vid. Try upping your bitrate to 10k or more, and make sure you click the watch in high quality button below your video in youtube. We put a slug at the beginning of all our youtube vids to remind people to watch in HQ.
Restart the computer….seems like an obvious answer but Mac users tend to forget that Mac’s are still prone to random bouts of stupidity, and like on PC’s, a restart is the great equalizer…..I use to run into out of memory errors and/or random stuff like that on an older machine we had. Restarting usually fixed everything.