Why can't Domain Users access second hard drive?

[rsbrux]

I have joined a freshly installed Windows 8.1 client to a domain controlled by Synology NAS. The Domain Admins group was automatically added the the Administrators (Administratoren) group and the Domain Users group was automatically added to the Authenticated Users (Benutzer) group as expected. Nonetheless, Domain Users and Domain Admins have effectively no access to the second hard drive in the system (as shown in Advanced Security Settings for the drive), although Administrators and Authenticated Users have all the expected privileges.
How can this be?
Could it be due to having installed Windows 8.1 in German?
What should I do about it:

1. Manually assign the needed privileges to Domain Users and Domain Admins?
2. Reinstall the OS in English?
3. or is there a more elegant solution?

 

[rsbrux]

Actually, I seem to be chasing a phantom; it appears that I can't trust what "View effective access" in "Advanced Security Settings" is telling me. It shows me exactly the same thing (no access privileges at all for domain groups) for the C: drive on both the Windows 8.1 client in question and on a Windows 10 client, for which I experience no access problems.

I will have to look elsewhere to find out why Domain Users have access problems on the D: drive.

Is there some other tool which can reliably identify and manage the access privileges of domain accounts on the client PC?