Yes, I am still using a WD "Passport" for downloaded movies. Copy on PC and connect the drive to TV's port. Yes, I know that I can stream. Yes, I have a good router. Never mind :-)
Oh, yes, I am still using Windows. Dear Mac users, I love you, too, but not as much as I love Bill Gates :) <jokes aside> Problem
"Windows cannot disconnect your USB drive". Close the programs, blah-blah-blah. Obvious
Well, closing File Manager's windows surely helps.
Waiting a bit after the file is "copied" helps, too. I guess, some portions of it are not flushed to the drive yet (who knows, it's Windows). Who cares?
My TV. Sometimes. Just does not see the files if the drive wasn't properly dismounted. Not so obvious The TASK MANAGER!
I believe, it monitors the disk activity. So, shut it down. Not obvious at all
Why? Who knows. I do not want to know. These days, I am using PHPStorm's Git or a command line. SourceTree is kept for some strange cases when I am not 100% sure what I just did... :-)