I’ve seen the exact same issue which I briefly corresponded with Bouke about. I modified TC of a QT movie that was a Canon 7D shot file. The file was on an external drive formatted NTFS.
Changed the TC and the file went bust. Same symptoms as yours. Size shrunk, no longer opens in QT.
I accessed a backup of the file, copied to my Mac internal, and TC change went off OK.
I think modifying the time code of a QT movie when the file is not on a local and Mac formatted drive makes the file go corrupt. I need to do this test on networked and FAT32 formatted drives just to check of that is the issue.
In the past I’ve seen issues when one sets a networked drive as a capture scratch. When one captures, the files come out without TC, or are corrupt.
I seem to remember reading up about how on a network information travels as packets. So when you open or write a QT movie you open the data out of sequence and if the TC data doesn’t come in in the right sequence with picture data, the file structure gets altered enough to lose some data and all timecode.
Just a conjecture. More info needed.
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Neil Sadwelkar
neilsadwelkar.blogspot.com
twitter: fcpguru
FCP Editor, Edit systems consultant
Mumbai India