It could be that your drive has spontaneously re-enabled permissions or ownership. Try this:

Right click (or Ctrl-click or two-finger click) the hard drive and select Get Info. At the bottom of the Info window will be Sharing & Permissions - in there, click on the yellow Lock icon, enter your password (or an administrator username and password), and make sure "Ignore Ownership On This Volume" is checked.

If you don't see your username in the section just above that, you may need to add yourself in with the + button, then make sure you have Read & Write privileges.


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