Can't access Folder Under Windows 2008

[Chris]

Hello I have a fileserver running w2k8. On the partition d: I've deleted all
groups except Administrators (full control) and Systems (FC)

The Administrators group contains:
- Administrator
- Domain Admins

My user is member of the domain admins group. If I try to access the
partition D:\ I get an error: "D:\ is not accessible - Access is denied."

I look at the effective permissions and there I can see my user with Full
Controll rights.

Has anybody an idea why I can't access the partition d:??

Thanks for your help
Chris