Also, when I swap the 4K+ with the 4K, playing the same 4K movies & 4K TV episodes from the same HDDs, play OK every time. That’s with swapping between the powered hubs as well.
That rules out the HDDs & powered hubs. If there was a problem with the HDDs or hubs, then those bad playing 4K movies & 4K TV episodes, should exhibit the same problems when played through the 4K, which they don’t.
The cause of the problem has to be with the 4K+