Some googling revealed, that
smartd of the smartmontools has a feature to skip checks when a disk is idle, but
smartctl does not. Some patching later, I had added a
switch, to make smartctl skip reading, if a disk is in
To my dismay, I found that the FreeBSD-specific code did not implement the CHECK_POWER_MODE function (see page 88 of the ATA spec), so I added that one, too.
Of course I have sent the patch with some explanation to the smartmontools developpers, but as usual, they don't even bother to respond to my mail.