Windows Server 2008 had a great change on Windows Server 2003, Extend Volume and Shrink Volume options are added to disk management right-click menu, but you cannot use the two functions freely. So when low disk space alert happens to you system partition, you need to do more to use new functions:
So if anyone told you to shrink D partition and you can extend C Drive, that's wrong. Shrink Volume is designed for reducing partition size and make new partition on unallocated space. Really cannot help to extend volume.
When you get unallocated space followed C Drive, you can extend system partition in Disk Management, or use diskpart command line to extend system partition. we have made detailed guides on each methods, you can click to read more:
Disk management alternative partition software: Partition Expert provides another solution on extending system partition of Windows 2008. You can do the extension without data loss: you don't have to delete D Drive or any partition to make space unallocated, or convert original file system FAT32 to NTFS. We'll show you how to easily extend system partition in Partition Expert：
Here are the screenshots of how to extend system partition in Windows 2008. You can take a look if you haven't done the software. In fact, it's so well designed that when you run it, you already known how to use it.