That account is the one that had rights to the files on the old system. What happens when you try to take ownership or change rights on that folder?
D&S on properties - security -advanced - Owner it shows my current login as Administrator. And Administrator has full privileges.
I just noticed that on the other folders unknown user does not have any privileges...but has special privilges (that box is grayed out) with a check in it.
Only difference i can find between the folders.