Prevent spin-up of USB HD after upgrading to FreeBSD 15

[Martin]

I have an external Toshiba USB drive that automatically spins down
after about 1 minute of idle. After upgrading to FreeBSD 15, the drive
keeps spinning up again about every 5 minutes even though it is not being
used and isn't mounted. This didn't happen in FreeBSD 14.3.

The drive is running ZFS.

I have monitored both /var/log/messages and daemon.log using tail to
see if anything happens, but nothing goes on in those logs when it
happens.

How do I track down what's spinning up my drive?

[Martin]

I have just found out that if I export the drive rather than
just unmount then it doesn't happen. So I am guessing that
it's something ZFS specific/related.

[Martin]

I have tested the same drive on the same machine using Artix Linux
and the drive stays spinned down.

I have used top -s 0.1 in order to see if I can get a glimpse at
something which might run, but nothing is started. And as mentioned
in the previous mail, nothing is happening in messages or daemon.log.

I am beginning to suspect that it's the ZFS kernel process thingy
something which is doing this?

I do not want this drive to keep spinning up every 5 minutes.

What has changed from 14.3 to 15 that might cause this annoying
behavior?