AirPort Extreme A1408 + Time Machine

To somewhat expand Tesserax's reply ..

There are two issues..

Reliability.

For some people this setup worked fine for others it revealed a problem in Time Machine that it would corrupt itself once every 4-8 weeks and start over. The issue is still there when you try and use some other non-Apple targets which supposedly support Time Machine.

Speed.

USB drive plugged into USB on any airport extreme or time capsule for that matter.. is slow.. glacial. It is not a big issue for very small backups once the full backup is done but you will find it sure is slow.

You could, but Apple only supported Time Machine with the "tower" 802.11ac Extreme model.