If the switch for detecting the locked state is stuck down, then it will think every disk is unlocked, since usually the tab on the disk will press it down.

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Well observed. Which means that cruft and dust (wonderfully conductive stuff) were the likely causes.

I'm sure that I have mentioned this before, but here goes... A web server on a low end PowerMac spontaneously rebooted from time to time. The cause was a bundle of fluff inside the Mac and surrounding the external reset switch. Depending on the weather (literally), the fluff acted as a switch.