Hard Disk 20: No Multiple System Folders

The "Macintosh Update Hard Disk 20" documentation which accompainies all
HD20's states:

Disregard any references to multiple system files on the Hard Disk 20 in
your owner's manual. Apple does not recommend that you have multiple
system files on the Hard Disk 20.

Instead, you could configure and "startup from" other volumes containing
the applications, system files and fonts you want.