Apple Macintosh System 7.5 - Manual (Page 107)

If your computer does not start up properly or behaves erratically, you
probably have an incompatible extension or control panel.
This appendix tells you how to remove startup programs that are
incompatible with System 7.5.
Verify the problem
To verify that you have a compatibility problem, follow the steps below.
1 Start up your computer, while holding down the Space bar.
Holding down the Space bar opens the Extensions Manager control panel
during startup.
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Appendix A
Testing for Compatibility