attaching USB drive to Duo v2

[md96]

I have a ReadyNAS Duo v2. I'm planning to attached a backup drive via USB (3.0 hopefully). I'd like something that spins up only when a backup job runs, but otherwise is asleep/not spinning.

Can I attach a USB 3.0 drive that is powered by the USB (e.g., WB passport)?

If so, will that drive continuously, or only when the backup job is running?

Or, if I want something that sleeps when not used, do I need to get a powered USB external drive (e.g., WB My Book)?

Thanks!