I don’t fucking know why I can’t eject USB hard drives. I installed the SysInternals apps, and best they can tell me is that Dropbox is fucking with the drives. I explicitly told Dropbox to not fuck with USB drives. I don’t know who’s lying, I just want whoever is fucking with the drives to stop fucking with the drives, OK??? OK.
Have you tried turning off thumbnails? In my experience when it slows down on large directories like that it’s either trying to generate thumbnails or index a bunch of new files (which is also a possibility in your use case)
A great thought on the thumbnails, but the behavior exhibits even when using command-line tools.
Indexing is off for the entire drive, I can’t even begin to imagine what kind of mess that would make if it was on.