I'm getting "can't add volume. Unknown error occured" when I try and add my Mac hard drive to the privacy settings

I don't know if it will solve the problem, but the first thing I thought of was disk errors. Boot into Recovery and repair the drive. Also, go in an repair permissions in Disk Utility afterwards.

Also, try deleting the com.apple.spotlight.plist from your username/Library/Preferences folder.

There are Terminal commands you can use to re-index spotlight, but I would think this is indicative of something more than Spotlight.

Launch the Console application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad. Click Utilities, then Console in the icon grid.

Make sure the title of the Console window is All Messages. If it isn't, select All Messages from the SYSTEM LOG QUERIES menu on the left. If you don't see that menu, select

View ▹ Show Log List

from the menu bar.

Click the Clear Display icon in the toolbar. Then try the action that you're having trouble with again. Select any messages that appear in the Console window. Copy them to the Clipboard (command-C). Paste into a reply to this message (command-V).

When posting a log extract, be selective. In most cases, a few dozen lines are more than enough.

Please do not indiscriminately dump thousands of lines from the log into this discussion.

Important: Some private information, such as your name, may appear in the log. Anonymize before posting.