VHD and VHDX are virtual hard disk file formats. They package a disk, or part of one, into a single file that can be mounted by a host operating system or hypervisor. VHD is the older format; VHDX is the newer Microsoft format with better resilience and support for larger disks. In managed Windows environments, these files often store profile containers, user data, or application state that is attached at logon and detached at sign-out.
In cyber security, VHD/VHDX matters because the same mechanism that makes profiles portable can also make problems harder to see. If a container fails to attach, users may get a black screen or missing settings even though the machine is running. Defenders use these formats for isolation, backup, and fast recovery, while attackers may hide files inside mounted images or use them to move data between systems. Monitoring mount activity, file integrity, and profile attachment logs helps distinguish a storage issue from a compromise.