…if the external drive is connected via SATA its probably ok, an external USB drive connected to the same USB controller may be problematic, since this doubles the load on the controller. FireWire is a different story, depends where the controller is physically connected. I’d really recommend to prefer an internal drive, at least for a first try.
And if you didn’t try a “short cable”, how long is the current one?
The Qu is directly connected to the Mac (without a Hub), correct?
Is there anything else connected to any USB port (i.e. webcam)? Mice, Keyboards do not count.
Single track should work with any DAW.
On Mac there’s no need to install any driver for the Qu, except you intend to use the HUI protocol to control your DAW from the Qu faders.