VHD and VHDX
VHD：Virtual Hard Disk is a file format which represents a virtual hard disk drive (HDD). It may contain what is found on a physical HDD, such as disk partitions and a file system, which in turn can contain files and folders. It is typically used as the hard disk of a virtual machine. The format was created by Connectix for Connectix Virtual PC product, which was later acquired by Microsoft in 2003, for what is now known as Microsoft Virtual PC. Since June 2005, Microsoft has made the VHD Image Format Specification available to third parties under the Microsoft Open Specification Promise.
VHDX：VHDX is a Hyper-V virtual hard disk (VHD) format found in Windows Server 2012. The main advantage of switching to VHDX is its increased storage capacity of 64 TB (terabytes), instead of VHD's standard storage limit of 2 TB. Other advantages of VHDX include file corruption protection and the ability to create differencing disks. VHDX files are not compatible with previous versions of Windows Server. However, the conversion process from VHD to VHDX is simplified with Windows Server 2012.
Note: Win 8/Win 2012 supports VHDX/VHD, Win 7/ Win 8 supports VHD. From the version of CCBoot v3.0 20140827, CCBoot supports VHDX as image format and write back file format. If the size of game disk and image is more than 2T, CCBoot will use VHDX as write back file format. If the size of game disk and image is less than 2T, CCBoot will use VHD as write back file format.
For more details please refer to http://searchservervirtualization.techtarget.com/tip/VHD-vs-VHDX-Virtual-disk-format-differences.