I have an Airport Extreme (5th gen) connected to a WD 4TB (usb). I'm trying to use it with Time Machine in the same way I used to do with my first gen. Time Capsule.
The problem is the 5 GHz network. If a Mac is using 5 GHz network and is starting to backup, the network will crash in a few seconds. The green light on the Extreme will be still on but the wireless network is gone. Only way to get it back is unplugging the Airport Extreme.
When a Mac is using 2,4 Ghz, there is no problem.
I've got a MacBook Pro and a MacBook Air and I'd like to backup both. Sometimes the Macs are using 2,4GHz and sometimes 5 GHz. Usually they are first on 2,4 GHz but then they change radio mode to 5 Ghz that is faster when they are "talking" to each other or to Apple TV. It's possible to make them change from 2,4 to 5 Ghz by turning Airport-marker off and on. Usually the Macs change from 2,4 GHz to 5 GHz that way. But I don't how to do the opposite or even force the MacBooks using just 2,4 GHz.
I have also an Airport Express extending my network. I've connected the airports in "the new way" so that both devices are using an ethernet cable and have identical setups. This is working very well. If I carry a MacBook thru my place, it will use the better signal. Near the Extreme Mac is using mainly 5 GHz and near the Express it changes to 2,4 GHz. Signals are stronger than extending wirelessly.
When Time Machine freezes Airport Extreme, "Express" disappears from Airport Utility menu. I suppose this happens because the Extreme was creating the network in the first place and the Express was extending it? Although after everything is installed ok, they both are "creating network".
Any ideas why TM doesn't work with 5 GHz?
The issue you are seeing, most likely, is not due to the 5 GHz radio band, but instead to the fact that Apple does NOT support Time Machine backups to HDDs attached to the 802.11n AirPort Extreme Base Station's (AEBSn) USB port.