Something is accessing my HDDs but what !?

Background -

• I have a Helios64 NAS with 4 x HDDs running Armbian 25.11.2 with kernel 6.12.58.
• The HDDs each have a ZFS partition but the partitions aren't imported.
• The OS is running from an sd card and logging is to ram.
• Power management and spin-down on the HDDs is disabled.

Despite all this *something* is causing the heads to move every second or two.

iotop shows no disk activity . lsof +d /mnt/sda etc. shows no open files and fatrace shows files in /usr and /var but nothing on /dev/sda etc.

So, whilst it doesn't look as if any files are being accessed the HDDs are still doing something. The question is WHAT and how do I stop it ?

Any ideas ?

Edited May 7May 7 by SymbiosisSystems