Thanks Tsaukpaetra! How can I set this value on the Truenas interface?
However, I found the culprit of the constant HDD activity: it was the SMART monitoring! I turned it off, and right then the HDD went silent! I’m sure it makes some sense for the experts but it is quite counterintuitive that a monitoring feature would constantly stress the drive when there is absolutely no workload whatsoever.
For future reference:
Storage > Disk health > Manage Disks > Select the disk > Edit > untick “Enable SMART”
I will optimize settings later for the security/quietness tradeoff however, I’m very pleased with it for now. It is dead silent finally! 