Couple of things come into play when accessing files. Permissions and labels.
Labels have to do with your current rights elevation while permissions are what you are allowed to do with directories and files.
Labels are low, medium and high. Most everything runs at a medium privilege label except for some browser processes run in low and the built-in administrator account runs in high.
So you need the current permissions as well as to be running in the correct label. Even as an user that belongs to the administrators group your user account still runs in medium for almost everything. The UAC prompt is a protection mechanism used to elevate a particular process to high to become privileged.
So you need to verify you below to a...