Thank ghe2001, Andrew, Reco, Hans and Thomas!i try bullseye for amd64, as Andrew suggest, it succeeds in copying 4G mp4 video
transfer speed is slow, i have to wait for half an hour after entering cp command
and i can't check progress by running "ls -l" in other terminal ("ls -l" shows new file hasn't been created)actually mplayer can play mp4, there's some error msg when i resize window, but it's not important
surely it can't play incomplete mp4 file (2G instead of 4G)