There are two ways to do what you want:
First Method
------------
1) Interrupt the boot process by pressing Control-Reset while the
system is booting. This will give you the "]" prompt.
2) Type "PR#6" to tell the computer to start up from the controller
card in slot 6. (If you're starting up from a 3.5-inch floppy disk, use
5 instead.)
Second Method
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To do the same thing through Aristotle, simply create a BASIC program with
the following line:
10 PR#6
Save this program and set it up as an Aristotle menu item. The problem
here is that you must start up all the way into Aristotle before you'll get
back to the ability to boot from a local drive. Control-Reset will give
you the ability to boot from a local drive much sooner.