Originally did you see two 4TB drives in Windows? Im guessing (and assuming since you didnt mention data loss) it was set as a mirror, so the 2x 4TB showed as one 4TB drive in Windows.
You replaced the dead 4Tb with a 6TB and the WD is using only 4TB of the 6TB to mirror the other 4TB drive.
Sounds like cache running out after a bit of transfer?
On the windows side you can try turning off (maybe) remote differential something, forget the exact name.
If it's a windows explorer copy your doing, try Robo copy, or Robo copy gui
Ive used diecutstickers.com 4x so far for projects at work, very good, not falling off after 6 yrs, I sent an initial request, they assigned a guy for me to work with directly, made a graphics examples, sent a few emails back and forth, then got a example done.
I had thought of trying to use...
Set inform but maybe try the ip instead of name?
Id lean towards it getting dropped due to age.
Maybe for the heck of it, for up an older version of the controller and see if it adopts there?
edit - square body AP...might have been dropped...