Permissions on existing drives after install

>I did search around but could not find a close question. With Vista 64 bit,
> installed to 2nd of 3 partitions on first sata drive. I also have a second
> sata drive all one partition, an IDE drive one partition, and a usb drive
> with one paritition. All are NTFS. All where accesseable and had
> permissions
> with a copy of XP with the user group of PCname\Administrators having full
> rights. Now I installed Vista64 and my user is in the Administrators
> group.
> Vista works fine and can access the first sata drive with 3 partitions and
> all partitions work. The 2nd sata drive say not accessable. The IDE says
> not
> accessable and the USB drive works fine. I was able to fix by adding my
> user
> with full rights to both non-working drives. BUT, my question is why did I
> have to do that when this user is a member of the PCname\Administrators
> group
> in Vista. While typing this I noticed that the 2 non-working drives do not
> have the PCname\Users group in the list. But again, why would it need to
> be?
>
> Thanks for the time and effort!