Excerpts from netbsd: 26-Jun-98 Re: kern/5654: boot hangs d.. "Simon J.
Gerraty"@quick (1176)

> I PR'd this because problems with a non-boot disk should not prevent
> the system at least getting to single user. 

It's intended that all hard disks are read from
during autoconfiguration to find the boot device. 
There is not much correlation between the BIOS
device number of the disk and the SCSI ID or whatever
physical attachment parameters. Therefore, all
disklabels are read to match the disklabel parts passed
by the boot program.
That's not elegant, but one has to blame the "firmware"
for it because it doesn't provide useful information
about the boot device.

So far to explain why it tries to access the disk at all
at this point.

best regards
Matthias