The users with the "unknown" title can normally be traced to previous users. The issue is often caused due to the way that NTFS stores data and the access permissions for the data.
Say you have some data (files/Photos/Etc.), and you give permissions for a user "Fred" to access it, the system does not see this user as "Fred" but as a long series of numbers known as the SID.
The SID is unique for each user, on each system, if you move the data to a second system, (even one with a user called "Fred"), the system will not recognise the SID of the first user and thus show it as unknown, (or it may actually show a SID reference S......).
This will happen on completely new systems when accessing data created on a previous or Just different system as the NTFS Security Identifier is saved as part of the file header
This can also occur if you have a multiboot system, that uses NTFS Security (NT/Win2000/XP/Vista/Windows 7), that access the same data