When building out a new VM I’ve occasionally created a second disk for holding non-system files and temporary data; this disk is easy to blow away every so often and saves the hassles of defragging and compressing.
I recently built a Windows Server 2008 R2 VM and created a system drive and secondary drive in Hyper-V manager before installing the OS. During the OS installation process, I created a new partition and noticed—but thought nothing of—the fact the installer didn’t nag me about creating a reserved system partition as well.
Carrying on my merry way, I eventually found the secret system reserved partition on my “scratch” disk (d:\) when I went to format what I expected was a completely unallocated disk. Bummer—I’m not clear on the ramifications of simply deleting this reserved partition but it currently means dragging around an additional 6GB of D: drive and being unable to blow away my scratch disk.