MKV file problem

Recently I’ve got this mkv movie file,the original file was mkv 3,8GB 1080p dts,then because I want to do some experiment I demuxed the file,then converted the video file using any video converter so the size changed from 3,8GB to 8,9GB then remuxed it with the original audio file(dts) using mkvtoolnix,it plays perfectly on my pc and any other pc,but everytime I want to back it up to my ext hard drive (WD Elements usb 2.0) using teracopy pro,the transfer seem so smooth,and I always recheck my ext hdd that the file didn’t corrupted,problem is everytime I try to hook my ext hdd with my wd tv live (firmware ver 1.06.15_v),the file can’t be recognized,then I recheck the file size suddenly changed from appr 8GB to some several kilobytes only,but this problem only happened with this file,the other movie files (other experiment) works fine,any idea anyone?thanks and forgive my bad english

Anyone with solution please?

Update: I just repaired the mkv file using meteorite,then copy the file to my other wd elements ext hdd,it can be played in wdtv,but with lagging,then I copy it to my old wd ext hdd(the troubled ones) it showed the same situation as before,can’t be recognize,so I predicted that the problem is within my old wd elements 2TB ext hdd or the teracopy pro software,maybe it have some copy history record or something,tomorrow I will try to erase the remaining garbage file in my old hdd,using revo uninstaller evidence remover,but still hoping that any of you could give me idea,thx