I just experienced the same problem on my MacBook Pro. Here are a few additional details that may help someone else who is searching for a similar solution: If you have a broken startup disk icon that looks like a light switch with chopped off letters at the bottom, the fix is simple. Go to the library folder. Open the preferences folder, and then the system configuration folder. Drag the preferences.plist file to your trash and delete it. Now your startup disk icon should look normal (icon with question mark in the middle). Apparently, plist files regenerate when you delete them.
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It actually fixed itself after the latest OS update. Other than waiting, I'm not sure how to fix it.
deleted a few preference pane plist files in my caches folder
I fixed it,
deleted a few preference pane plist files in my caches folder
Hey neato! Thanks for that, it's been bugging me for a while