I’m sorry, I forgot to put people in Cc, as stated in the documentation. :-(
I’ll send my answer to the report again:
I’m also facing this problem.
As Nia said it's since the 9.0-RELEASE.
I also faced this on 9.0-STABLE (my current setup) and -CURRENT.
Sometimes this error appears along hdafg0: invalid child range.
As Nia, previously stated, they appear mostly on reboot.
Here the link from pastebin with my dmesg: https://pastebin.com/7YtU75Zr
NetBSD 8 worked just fine in my experience. I never faced such issues. I also tried to stress my installation by rebooting it as soon as the system went up, but everything always went fine.
Luca De Pandis