>> You definitively need to avoid the NetBSD disk label.
> I don't understand how I put a NetBSD disk label on there in the
> first place.  The only disk/filesystem commands I used in NetBSD were
> newfs, mount, installboot, and umount.  Do you know if any of these
> cause a label to be written to disk?

I think newfs tries to rewrite the disk label, which is likely to cause
an updated label to be written - which may well create a BSD label.

It might be possible to use newfs -F to avoid this; see the manpage.

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