I have been using an Airport Extreme (ac), model A1521, purchased 3 months ago. I've have it hooked up to a powered USB 2.0 hub with four external hard drives, all externally powered, each from its own power supply. 4 different machines in our LAN back up via Time Machine to these drives. I left town for a month, disconnecting the 4 drives and their hub both from power and from the AE. When I returned, none of the external drives would mount through the network if connected to the USB hub, but each would mount if not connected via the USB hub, that is to say, connected directly by USB to the AE. The USB hub had failed before, so I bought a new one, a USB 3 hub. Both the old and the new hub are powered, and by the test of charging an iPhone, each is receiving power (only one emits light when powered). Neither one will allow a hard drive connected to it to mount via the network. It does not matter whether I look at the network from my Mac Pro, or my MacBook Pro, what I see is the same. (I used the same setup successfully for 7 years with an 8 year old Time Capsule.) I, of course, completely powered down AE, cable modem, all connected hardware, each time I made a change. I have tried connecting the drives individually to the hub with the hub connected to the AE. Not the new hub, not the old hub will work with 1, 2, 3, 4 external drives connected. I've done a lot of this kind of troubleshooting before, and I'm surprised that I'm not making progress. Is there some network cache that I can flush? Should I turn every piece of equipment in the entire network off, and then power it back up, starting at the center and going to the periphery? I've spent a few hours on this, and some money, with nothing to show. Suggestions? Questions?