How to remove unknown accounts from Windows

When checking file/folder permissions you may encounter some strange account names with question marks. These are "Security Identifiers" (abbreviated as SID; wiki definition: unique, immutable identifier of a user, user group, or other security principal), possibly left from removed accounts.

While they are mostly harmless you may want to remove them to prevent annoyances. The easiest way (apart from using SubInACL on command line) is to delete them from root folder (sub folders inherit account settings). You can do this by right clicking a folder and then from Security tab you can change permissions by clicking “Edit” button, and from there you just need to remove the “unknown account”.

Some folders may give you an error like the one below while trying to remove the account, in that case you first need to change the account owner.

To change owner click to “advanced” from “security” tab and click “change” (the one with a shield symbol).

Don’t forget to select “Replace owner on subcontainers and objects” and “replace all child object permissions with inheritable permissions from this object”.

After that you can remove the account name.