RT-AC68U disc random spin-up

I have the RT-AC68U router on Asuswrt-merlin, with 2 external usb hard drives attached. The spin-down feature of Merlin works great, but I also notice that my hard drives would spin-up randomly for no reason. I've checked all my networked devices to see if any of them was accessing my drives without me realizing it but cannot find anything. Tried to see if there's anything unusual from my system log, but was not able to make sense of much because of my limited networking knowledge.
Here's a...

RT-AC68U disc random spin-up