The same with previous version, Windows Server 2012 provides built-in Shrink Volume and Extend Volume functions in Disk Management (DM for short). However, if you want to extend system reserved partition in Server 2012 DM, it is impossible, because:
As you see, Unallocated is generated on the right side of C drive after shrinking, Extend Volume is grayed out for system reserved partition in Server 2012 DM.
The only way to get right contiguous Unallocated space is by deleting the system C: drive, but you can't do this. Thus, you need 3rd party partition tool for Server 2012.
With partition software such as NIUBI Partition Editor, you can shrink C drive and generate Unallocated space on the left side, then extend system reserved partition with Server 2012 Disk Management or 3rd party software.
It requires server reboot to shrink or move C drive with any partition software.
Steps to extend system reserved partition on Windows Server 2012 (R2):
Step 1: Download NIUBI Partition Editor, right click C: drive and select "Resize/Move Volume".
Drag the left border towards right, or enter an amount in the box behind "Unallocated space before" in the pop-up window.
Click OK, C drive is shrank and Unallocated space is generated behind the system reserved partition.
Step 2: Right click the system reserved partition and click "Resize/Move Volume" again, drag right border towards right in the pop-up window.
Click OK, Unallocated space is added into the system reserved partition.
Step 3: Click Apply on top left to execute, done.
NIUBI is designed to work in virtual mode, real disk partitions won't be modified until click Apply to confirm.
If you don't want to shrink C: drive or there is not enough free space in it, you can get space from data volume (D:). Because D: is non-adjacent, before adding Unallocated space to the system reserved partition, you need to move it from right side of C to the left.
Watch the video how to extend system reserved partition in Windows Server 2012:
It is easy and fast to shrink C drive and extend system reserved volume, however, you'd better always back up these partitions and use reliable partitioning software. Both drives are where Operating System located and booting from, any error leads to system boot failure.
Many server administrators feedback in the forums that they experienced such disaster while resizing disk partition. It is easy to understand, all parameters of both drives must be changed correctly, all files in system C drive must be moved because the start position changed, system boot related files must be updated, too.
Nobody likes to see any issue, but there is indeed system damage risk. Better than other tools, NIUBI Partition Editor has innovative 1 Second Rollback technology. If any error occur while resizing partitions, it is able to automatically reverts server to original status in a flash. If it happens, server could back online without damage in a short time. Otherwise, you'll waste a long time to restore.
Furthermore, it provides Virtual Mode, Cancel-at-will, Hot-Resize technologies and advanced file-moving algorithm to help resize partition faster and more safely, learn more.