1. A non-transitory computer-readable storage media tangibly embodying instructions that upon execution by a processor cause the processor to: open a virtual disk file, the virtual disk file including a unique identifier identifying the virtual disk file and virtual disk extent data that forms at least a part of the data in a virtual disk, wherein a first unique identifier is stored as the unique identifier; overwrite the first unique identifier stored as the unique identifier in the virtual disk file with a second unique identifier in response to detecting an attempt to write information to the virtual disk file that affects how virtual disk extent data is interpreted; determine that the second unique identifier was written to a persistent copy of the virtual disk file; and write the information to the virtual disk file in response to determining that the second unique identifier was written to the persistent copy of the virtual disk file.