Using Diskpart to Expand my C:\ Drive

Pretty sure diskpart will not work on the C drive. You can try extpart, but it could corrupt the installation so have a backup ready.

Per the MS KB you cannot use diskpart to extend a boot partition or a partition with a swapfile on it.

Also; to extend any partition, you'll need free space right next to the partition (unless you go Dynamic disk, but if you value your data I'd recommend against dynamic disk due to the difficulties recovering from partition issues compared to basic disks). So if your current C-drive has the D-drive right next to it, you'd have to delete the D-drive before extpart can be used to extend the C-drive. If you cannot delete your D-drive, or it would be prohibitively expensive (time-consuming could be expensive), you could buy something like Acronis Disk Director Server. Currently it seems to be about US$ 609 per license (server), so that's not too bad if you cannot afford downtime (to back up the D-drive, delete the D-drive, resize the C-drive, and restore the D-drive).

Edit: obviously you still want to run backups prior to doing any of the options, just in case something goes wrong).

diskpart only supports extending the boot partition in Windows 2008. Anything below that, it will not work as it notes in the 2003, XP and 2000 section:

"Only the extension of data volumes is supported. System or boot volumes may be blocked from being extended, and you may receive the following error: Diskpart failed to extend the volume. Please make sure the volume is valid for extending"

:emotion-60: hello, http://www.extend-partition.com/extend-system-boot-partition.html This should take care of your problem. At least it did for me.