Confusingly, BT offers two different mesh systems with similar names. The black discs link to the Smarthub2 which becomes part of the mesh system. You get a single black disc free when you opt for Complete Wifi and, after linking it to the Smarthub, you can place this somewhere around your house to increase the Hub's WiFi coverage. However, if you want extra black discs you have to apply to BT and, I think, pay for them.
The white disc system ("BT Whole Home") operates independently from the Smarthub2 with a different SSID and, as has been said, you often (but not always) need to disable the Hub WiFi for it to operate well. The Wholehome package contains 3 discs, one of which has to be plugged into the Smarthub with an ethernet cable, the other are positioned around the house. Extra discs are easily available (at a price of course) and simple to add to the system. My rather rambling house with lots of thick walls needs 4 discs in total to give me good WiFi coverage.
