Microsoft uses and supports their VHD (Virtual Hard Disk) format on their virtualization products: Virtual PC and Virtual Server. You can get full access to the detailed specification online by signing the Virtual Hard Disk Image Format Specification License Agreement.
As you may have heard, Virtual Server 2005 R2 SP1 BETA2 (quite a name) is out now (read Jose Aguilar's post on it for more details) and it includes Offline VHD mounting. This feature was already planned to be included in System Center Virtual Machine Manager, but it is sure nice to get it now with Virtual Server. So what is the big deal about this? What Offline Mounting does is, it lets you mount your VHD files as if they were real drives on your server and it lets you do all the reading and writing you want.
Before this very handy feature existed, if you wanted to copy some files from or to your VMs you would have had to create a virtual machine, link it to the VHD, run it, and use the Virtual Network to make the transaction between the server and the VM. With Offline Mounting this task becomes quite a lot easier and you can skip a few of those in between steps. You can access Brian A. Randell's blog for more details on how to do this.
In the end, Offline VHD Mounting is all very nice but some of you may have noticed it brings up a bigger possibility, Offline Patching. If you have a company and you want to cut expenses on your Disaster and Recovery Sites this is a topic that should really gain your attention. Microsoft is definitely using Offline VHD Mounting as a message that they are heading towards an Offline VM Patching solution (Microsoft has hinted about this on a few ocattions). Since Microsoft's Virtualization Licensing policies only charge for those VMs that are actually running, you can picture the economic impact this is going to have in the future, your Back-up sites will not even have to run unless a disaster does happen.
Offline VHD Mounting is not just a nice new feature that will help you speed things up, it is also Microsoft's announcement on what's to come!