It won’t take up much bandwidth on your home network and has no impact on any ISP data limits … but transferring 850GB of data is gonna take a Loooooong time going through your modem/router 
Two alternate ways which should be quicker and not use any bandwidth.
I would try the “Import from USB” option Page 12 of the User Manual
Or connecting your Laptop directly to the My Cloud via an Ethernet Cable (this will bypass your modem/router not taking up any bandwidth from your home network)
