Here's how you can lock a file in OS X Mountain Lion by using the Get Info menu. 1. Select the file. 2. Press Command + I on your keyboard (or select File then Get info from the file's menu in the menubar). 3. Click the "Locked" checkbox under the General section of the Get Info menu. Your file will now be locked and modifications in the Get Info menu will be prevented until you unlock it.
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Use the Get Info window in Mac OS X | Daily Mac Tips