Apple Macintosh System 7.5 - Manual (Page 101)

If your Macintosh does not start up properly or behaves erratically, you
probably have an incompatible startup program (also known as a system
extension or control panel).
This appendix tells you how to remove startup programs that are
incompatible with System 7.5. It also tells you how to turn on startup
programs that were placed in the “May Not Work With System 7.5” folder
during installation and test them for compatibility.
Remove incompatible startup programs
To verify that you have a compatibility problem and test individual programs,
follow the steps below.
Verify the problem
1 Start up your computer, holding down the Space bar.
Holding down the Space bar opens the Extensions Manager control panel
during startup.
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Appendix C
Testing for Compatibility