Previously I hesitated from applying this solution since I’ve noticed some users claiming it didn’t resolve their issues, the others claimed they even bricked their own system after playing with those variables and by the fact Manjaro still didn’t implement it by the default since 2 years ago (when this discussion started). After your reply I gave it a try, rebooted my Manjaro and tried to copy the same 1.mp4 with the same USB drives and now I could normally watch my mp4 files on each machine I tried. The sync was triggered right on time, after each copy has been completed. That was an absolute success! 
Thanks for your valuable response, I haven’t noticed it on the old forum before! 
I wonder why this is not the default already. Cheers!
$ cat /etc/sysctl.d/98-dirty.conf
vm.dirty_background_bytes=16777216
vm.dirty_bytes=50331648