Some will cause it to crash on startup. Volumes formatted as HFS will mount, but the b-tree becomes corrupted causing it to become invisible in OS 9. Disk Warrior and Norton can fix these corrupted b-trees. The recently used items list is flaky about what it remembers, and I have found no way to clear it short of deleting a user and recreating them. Transparency of floating windows in OS X does not work with Classic.
Items become solid when over a Classic window or app.
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