The Windows 8.1 Update that released in spring has a new functionality called WIMBoot. This is a method of saving space used by Windows by placing system files on compressed WIM files. These WIM files are placed in a new partition which replaces the recovery partition.
NOTE: This will only work on a GPT disk.
Here is a GPT disk that uses the classic layout with Windows living in a standard partition.
Here is the new layout. Instead of storing the Windows files normally, they are in a compressed WIM in the images partition. The actual Windows directory simply has files that are in fact reparse points. They contain a small amount of metadata and then point to the location of the actual file within the appropriate WIM file.
If a file needs to be written to, it will be copied into the standard partition first. After that, that file will be accessed from the standard partition instead of being redirected to the WIM.
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