I am using diskpart to extend a drive that is actually a VHD. I've already extended the VHD. It's on Windows 2003 and the C drive doesn't contain the swap file and the available space is contiguous. However didn't see the Note about the Resource Kit diskpart for download is not for Windows 2003. So I did the extend using the Windows 2000 version. Not sure if this is the reason but Diskpart is sitting there now for about 15 minutes or so and it's only gotta extend by 10GB. Should it be taking this long? Am I asking for trouble now that I've used a Windows 2000 version of diskpart on a Windows 2003 machine (VM)?
In my experience a Diskpart extend is generally nearly instantaneous but I have to admit that I have only used it to extend volumes that are Windows data drives on VMware Windows VM's.
Diskpart for Windows 2003 Extend functionality is only for Data Volumes with the additional clarification that the volume being extended can't contain the system page file and that system\boot volumes may fail. My understanding is that the "may" in this case refers to versions earlier to Windows 2008.
As far as using the Windows 2000 version of Diskpart on a Windows 2003 server I wouldn't do it but in the situation you find yourself then running chkdsk should identify if it has caused real problems. If you were using the Windows 2003 version, and if by "C" drive you mean the System\Boot drive then I would expect it to fail with the following message:
Diskpart failed to extend the volume. Please make sure the volume is valid for extending
It depends on how fast the disks are. It will zero out all expanded disk space, so how long it takes to write and verify 10GB of data on your computer/server will be the largest factor.
All VHD files are the same format since MS bought Connectix some years ago (somewhere around 2003).