reducing maximum disk space

Hi and good morning,

I'm new to this community.

I have some issue shrinking my disk size.

OS: Windows Server 2008 R2

Workstation 10

The maximum disk size is 120GB.

Where windows has only 20 GB allocated, it is not possible to get this 100 GB back.

I've already tried to set it back manually... no chance

Manage / clean up disks (I read this is the new shrink function) ... no success either

Is there a way to reduce the maximum disk size without reinstalling?

Thank you very much in advance.