Not able to remove "Unknown User" in Sharing Tab!

You might be in danger of causing more harm than good? OS X is UNIX under its shiny GUI and there is an unknown user that is an integral part of the OS. This would be a POSIX user reserved by the operating system for use in special circumstances. This account usually prefixed by an underscore and has a uniqueID of 99. You can see the all user accounts - POSIX or otherwise - by launching Directory Utility. This app is in the System/Library/CoreServices/Applications folder. Launch it, click the Directory Editor icon, click the lock icon, authenticate and you should be viewing the Users folder in the Local/Default node. If the unknown user is listed with an underscore and it is the only unknown user you see then it is the POSIX unknown user account and you shouldn't attempt to delete it as you may cause the system to become unstable over time. If there is another unknown user without the underscore then you should be able to delete it safely in Directory Utility by simply selecting it and clicking the 'minus' icon at the bottom of the screen.

In my experience a file that has the _unknown POSIX user assigned is generally (not always) because the file has somehow 'lost' its original POSIX owner. It that sense it's then an 'orphaned' file needing to be owned. As far as I know ACL permissions would carry on as normal and should not be affected.

There is an "_unknown" user which should be left as-is (but shouldn't apear in your User list).

Run this command in Terminal, it will return a list of any user with unknown in the name

dscl /Search -list /Users UniqueID | grep -i unknown

You should only have one entry returned, with ID: 99

_unknown 99

If you have others, they can be deleted.. but I'd rather see your results before posting instructions.

If you have more than _unknown:99, post the results.

Jeff

Well something is putting it there and I don't see that user when searching for it in my sharing preferences pane.

However perhaps you're looking in the wrong place? I don't think I've ever used the File Sharing part of the Sharing Preferences Pane to administer the File Sharing Service in OS X Server? I've always used Server Admin (prior to Mountain Lion) and the Server.App since then.

Perhaps the unknown user has somehow added itself in the Server.App and that's where you need to remove it? When I say remove I do mean remove and not delete. In the Server.App you add users and permissions to shares you create. Removing users from the share using the Server.App does not delete those users.

Perhaps that's where you should be looking?