Hello,
I have a 2 TB Wd Passport Ultra connected to the 3.0 USB port of my AE6900. I also have a old aiport extreme (3 years old) connected to another Wd Passport Ultra on 2.0 USB.
You know what?
If I run Blackmagic Disk Speed Test and I select the remote HD on AE6900 I have a write speed rate of about 10MB/s, but if I test the other HD connected to the old airport extreme I have 30MB/s write speed.
That is strange!! I expected higher speeds thanks to the AC protocol and USB 3.0. Very disappointing...
I just ordered a new Capsule Time Machine and will check again the performance.