Yea I would test a large file and as soon as it buffers put a limit on quittorrent or something that could be reading and writing lots of data from those drives and see if that fixes the issue. I wish there was a better way.

Its a qbittorrent setting. you can set upload and download limits to turn on at specific times or when you want to by clicking the little speed dial.

So if you are not utilizing those harddrives for other uses such as torrents thn disregard.

I believe I have a very similar issue as you. It only occurs on remux/high bitrate 4k videos. Happens no matter the player etc. I currently fix this issue by putting on that limit qbit upload speed and everything plays fine then.

I am interested in what your solution is.