2.2 Header Section
The header section is the first structure on the disk and is the structure that is examined first when opening a VHDX file. The header section is 1 MB in size and contains five items that are 64 KB in size: the file type identifier, two headers, and two copies of the region table.
The file type identifier contains a short, fixed signature to identify the file as a VHDX file. It is never overwritten. This ensures that even if a failed write corrupts a sector of the file, the file can still be identified as a VHDX file.
Each header acts as a root of the VHDX data structure tree, providing version information, the location and size of the log, and some basic file metadata. Other properties that might be needed to open the file are stored elsewhere in other metadata.
Only one header is active at a time so that the other can be overwritten safely without corrupting the VHDX file. A sequence number and checksum are used to ensure this mechanism is safe.
The region table lists regions, which are virtually contiguous, variable-size, MB-aligned pieces of data within the file. These structure currently include the BAT and the metadata region, but they can be extended by future revisions of the specification without breaking compatibility with different implementations and versions. Implementations MUST maintain structures that they don't support without corrupting them. Implementations MUST fail to open a VHDX file that contains a region that is marked as required but is not understood.
The header section contains five items: the file type identifier, two headers, and two copies of the region table, as shown in the following figure.

Figure 3: Header section layout