Why do I get error msg: "The request could not be performed because of an I/O device error" when copy large files

NAS device is pluged in to the network on RJ 45 100 mbps switch port

I connect to device via share, If I copy small files it works ok, but as soon as there is heavy io and the files is getting bigger is gives error "The request could not be performed because of an I/O device error" in your program or explorer.

There is 900 gb left on the device, We took it back the the suppleir and they gave us anotehr device and still the same error.

Is this devices limited to small files and what is the size, does anyone else use this.

Thanks