Microsoft decided to make its VHD format available to third parties under a royalty-free license. We already saw PlateSpin, in this post, and Acronis, in this post, moving to gain benefits from this choice. More vendors will surely adapt their products to interact with the VHD format. The first category doing so will probably be antivirus, now eventually able to scan virtual machines HDs from the host OS. The specifications disclosed, which are going to be discussed at Microsoft ITforum 05 conference, include:

• Hard Disk Foot Format
• Dynamic Disk Header Format
• Block Allocation Table and Data Blocks
• Implementing a Dynamic Disk
• Mapping a Disk Sector to a Sector in the Block
• Splitting Hard Disk Images
• Implementing a Differencing Hard Disk
• CHS Calculation

You can apply to see details by signing the Virtual Hard Disk Image Format Specification License Agreement here. Update: The license agreement states that any product developed with VHD manipulation capability needs to be distributed in binary form only. Thanks to Robert Aitchison for the insight.