>> What am I doing wrong? Did I miss an important remark in the near past 
>> about this problem?
>
>  This is due to a mistake in the installation instructions that was not
>found before the release.
>
>  Since the ados filesystem is a read-only filesystem, you have to mount
>it read-only:
>	mount_ados -o ro /dev/sd0e /mnt/ados

BTW

I've just updated the INSTALL document on the ftp site and in the tree.

Chris.