I have strange issue with file permissions.
F.e. On Windows 7 under Virtual PC I have VM with enabled undo disk. Permissions for .vhd file of this VM grant full control to local Administrators group, of which I'm a member. I have checked in advanced properties of this file and confirmed that my user account (it domain account) has full effective permissions. Ok, this expected. However, when I try to applying changes in virtual machine undo disk, I receive an error message saying that one of master or undo disk for this VM is busy (I have checked, this is not true - both files are not open by any process). Notice, that this error message doesn't mention anything about permissions at all! However, after explicitly granting to my domain account full permissions for this VM vhd file (which doesn't change my effective permissions, as I already have full permissions as a member of local administrator group! I have checked it!), I could apply with any errors changes made to VM undo disk!
Described here case of applying changes to VM undo disk is only one example of many other similar situations, when granting appropriate permissions to groups of which I'm a member is not enough and I have explicitly grant permissions to my account too. I have observed this behavior under Windows 7 and Windows Server 2008, but not on Windows XP.
(P.S. In any of such case, none of permissions entry uses explicitly denying permissions)
Could someone explain me where is the problem?