So for how long would my NAS be able to run autonomously? 

Carambola2 data sheet tells me power consumption is 0,5W. This boils down to a current of 100mA 

The portable hard drive, based on a review, consumes anywhere between 1,5 and 2W. Lets stay on the high side to be safe and assume an average usage of 2W or 400mA.

Our power bank has a rating of 10400mAh, thus 10400 / (100 + 400) ≈ 20 hours (mAh / mA) 

What's more, the Xiaomi bank supports a thing called "pass through charging". This means it will be able to supply the NAS with power even while being charged. It just takes what it needs to charge it's battery and leaves the rest available to our SOC and hard drive.

This means I can leave it hooked up in the car and take it with me as we go for a walk.