You could notice, after an upgrade to Windows Server 2012 made via an “in-place” upgrade rather than a reinstallation, the total amount of free disk space has lowered is really notable.
The reason of this behaviour is simple: the update to Windows Server 2012 keeps a complete copy of the previous operating system, and this is easy to check by looking at the content of the OS disk:
The folder called Windows.old holds the previous operating system, and is some situations its size could be really huge. You can completely remove it, but a simple delete operation is not easy because there are several nested permissions and ownerships.
An easy anc clean method is to use Disk Cleanup, even if it requires a reboot. This tool at first sight is no more available on Windows Server 2012, but in reality you only need to install it. As happens often with Microsoft products, there is no clear sign of the software inside the application management control panel, but you would need to install “Desktop Experience”. For sure this is not a nice design choice.
So, you would have to go into Server Manager, then “Add roles and features” and select to add the above mentioned component:
Once the installation is completed, you should reboot the system. Once the tool is available, you can finally start Disk Cleanup:
In my example, the tool is correctly showing the 22 GB I identified before, and it’s correctly confirming they belong to the previous Windows installation. So, we can start the cleaning procedure and free all the disk space.
As you can see, the used space has been reduced from 108 to 85 GB, that is exactly the 23 GB occupied by the Windows.old folder, that now is not on the disk anymore.