The biggest HS-DHTGL/R5 is the HS-DH8.0TGL/R5, so the issue is not one of drive size.
Can I assume that this 400GB folder has various subfolders with different sized files in it?
Do some of the folders have many (thousands possibly) of small files in them, while others have just a few large files in them?
A folder with 100GB of jpg/gif images will copy much slower than a folder with 100GB of ISO images from CD and DVD media, so the likely reason you see the slow down is because of hitting some folders with lots of smaller files in them. When the copy process gets to some folders further down the list with bigger files in them, then it should speed up again.
The highest capacity Terastation Live, ever sold by Buffalotech USA is the HS-DH4.0TGL/R5. We don't even have a part number for a 6 or 8TB TS Live in our database (never had a part number, so never manufactured it in the USA) - drmemory