What is accessing external drive?

I have a 2012 MBA and am running a USB external raid enclosure attached to a 5th gen AirPort Extreme. When the MBA is asleep all is normal, but when it's awake the lights on the nas drive are blinking a lot, and it's also doing it with lid closed and plugged in (I assume it's making time machine backup- but that's set to a different NAS). On the nas in question is just some stored data and my iTunes library. iTunes isn't running right now on the MBA.

How can I find out what process is accessing the airport drive? I'm new to os/x so the newbie answer may be the right one.