I have a Nokia Lumia 920 and Nokia Lumia 1020 phone and have a problem copying or moving videos to my computer using Windows 8 or Windows 8.1. The problem has been existent with the Amber update and the Black update. If I have multiple personal mp4 videos taken with the camera and I select all mp4 videos from Windows Phone\Phone\Pictures\Camera Roll, then copy or move these mp4 videos from the phone to my Windows 8 or Windows 8.1 computer, only videos under 100MB will be left in a state healthy enough to be played successfully on the computer. In other words, mp4 videos over a 100MB get corrupted when selecting multiple videos and copying (CTRL-C) or moving (CTRL-X) from the phone to the computer in the one process. The videos over 100MB are not able to be watched on the computer and are lost. Beware, I have lost some good family videos and not happy.
The only solution to this I have found is to copy or move over all the videos under 100MB at once from phone to computer, then copy or move videos greater than 100MB one by one (not in a group). Copying the larger videos one at a time is a work around and is the only solution I have found to avoid losing my personal family videos.
Crazy why it hasn't been fixed yet, as it happens for me on two completely different phones (920 & 1020), two completely separate computers, two completely separate operating systems (Windows 8 and Windows 8.1) and two completely separate updates (Amber and Black) - sure others too would have the same problem.