I recently lost permission to one of my folders (Pictures folder) and the all contents inside. I still have Read and Execute rights, but I lost all others. I don't know if it has anything to do with Homegroup sharing because I recently set up a Homegroup and shared the folder. I removed the folder from sharing but it didn't fix the problem.
Something odd I noticed was in the Properties->Security tab, where it lists all users who have access to the folder, it lists Account Unknown followed by a long string. Normally it lists [my name]\[computer name] or SYSTEM. I have a feeling that the unknown account is actually my account, but somehow the computer is not recognizing that fact. I tried to manually edit permission to give the unknown account full control but it wouldn't let me do it (access denied error).
I could just live without it, I can probably create a new pictures folder and move all the files, but I'd like to know how to fix the problem.