Aladdin Systems writes: "There is an incompatibility between StuffIt SpaceSaver 4.5 and Mac OS 8.1 which will not allow a Mac using the new OS to completely startup when the SpaceSaver control panel is enabled. The incompatibility is due to another set of significant changes to the Finder for HFS support and other changes to support multi-threaded activities in the OS.
Aladdin Systems is aware of the problem and is working on a solution. Until then, users of SpaceSaver need to disable the Control Panel (turn it off), or postpone upgrading to Mac OS 8.1. Aladdin customers can access SpaceSaver compressed files via StuffIt Expander. If the choice is made to expand all SpaceSaver compressed items, make sure that there is room for all the expanded files on the hard disk where the files are to be expanded.
Users of StuffIt SpaceSaver 4.5 should check the Aladdin Systems website for updated information."