OK, so this has turned into a complete cluster-cuss.
The issue turns out to be some permissions/ownership nightmare when copying files from an OSX machine to the Windows machine. All sorts of weird and apparent random stuff happening where files and folders on the Windows machine think they're in use when they're not.
I've scoured the web but there's a seemingly endless list of MAC/PC networking/server permissions/ownership open/in-use issues and it extremely hard to determine which is the one I'm experiencing.
I've never had this before, have happily moved files from mac to PC and back again for years.
It seems like a huge ask but does anybody have any recent experience with this open/in-use issue and a fix ?!?!
in on OSX 10.11.6 and Windows 10, machines are connected via an ethernet switch and I connect to the PC from the Mac via SMB
The last line of your second post might help. I remember some other folks having issues with SMB in the past.
I'd recommend searching the forum.
Here's a pretty relevant-looking thread: After effects - SMB - Extremely slow read and write.
Are you still facing this problem? If not, let us know how you solved it. If so, please let us know so we can assist you further.