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.
Many files copied to an external disk are cached, and not written immediately unless you run the “sync” command.
So when you press the “eject” icon, on most frontends a notification will tell you not to unplug it right away, once the write is complete it will send a “it is safe to remove the drive” message. This can take a couple minutes if you are writing a big file to a slow drive.
By default Windows disables file caching on external USB drives. It should be writing those files directly. That doesn’t prevent a program from locking a file or folder that it is using though.