Hey, we have the same problem on the latest TrueNAS release. 

System, asked us to collect and send a core file. 

and here is Top : 

@Damian Szidiropulosz please revert this https://jira.ixsystems.com/browse/NAS-103096 for both 12.0 and master, this was a useful feature that was helpfulin situations like this (when the drives support it of course)

This is collectd polling for drive temperature every 5 minutes using S.M.A.R.T. This interval cannot be adjusted.

We have "Force HDD Standby" checkbox that will force HDDs to sleep after a specified amount of IO inactivity (yes, these S.M.A.R.T. polls prevent some drives from sleeping)

It's weird for a drive to unpack and start spinning when it receives a S.M.A.R.T. information request.

I also believe this isn't letting the disks idle (although I don't idle my disks or spin them down), I believe it's not letting the WD PWL idleout.

I guess it's not really a problem, but more of trying to figure out why it's happening. If it is the SMART process, then can the interval for polling be adjusted. I was under the impression that the HDD's would only be read from if there was another using it.

Also, another user in the linked thread pointed out that the every 5 minute reads don't occur on TrueNAS core, but only on TrueNAS SCALE, on the same hardware. That lead to another user suggesting that we log it as a potential bug. ** 

I only noticed that it's a problem on my particular hard drive (EXOS X16) because it parks the head after 2 minutes of in-activity. Then, 3 minutes later, it unparks the head and starts reading. I've since adjusted the power settings to not park the head at all. It's also noisy when it starts reading every 5 minutes.