>     > one fdisk pseudopartition (for MBR bootloader to
>     > work) is created like this...
>
> Thanks. I remembered late last night that disklabel (on i386
> at least) created BIOS level partitions, and that fdisk
> subdivided these further for use with BSD. I used 'disklabel

no. disklabel divides for BSD and fdisk for BIOS.

> -I -i wd0' as per your suggestion, and then wrote the label
> to disk with the W command.
>
> I then ran fdisk, not sure whether that was a good idea or
> not (can I make a filesystem on a disk that has just been
> labelled?).  That prompts my next question - how do I make a

don't need fdisk if you do only netbsd. please remove mbr with
dd if=/dev/zero of=/dev/rwd0d bs=512 count=1

run disklabel -i wd0 and do only W once again

> filesystem?  I tried 'man mkfs' without success.

newfs -m 5 -b 4096 -f 512 -i 8192 -c 50 /dev/rwd0a

should be OK for 170MB disk you have (you may omit every parameter except
/dev/rwd0a to use defaults)

> I won't be installing NetBSD onto this disk. I'm just trying
> to make a data disk that I can mount onto the disk that
> already has (some of) NetBSD installed.
so you don't even need fdisk and don't use parameters to newfs (or use -m
0 -b 8192 -f 1024 -i 32768 assuming you will store large files)