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A virtual hard disk is a file that an operating system or virtual machine can use like a disk drive. The file stores the disk’s contents; when the file is attached, the system can access the virtual disk and use its file system. Microsoft calls the file on physical storage the backing store.
How a virtual hard disk works
A virtual hard disk separates the disk a system sees from the physical storage holding its data. Instead of requiring a separate physical drive, the virtual disk is represented by a file on an existing storage device. That file can contain a native file system and support ordinary disk and file operations, as Microsoft explains in its VHD overview.
The file itself is not the same thing as an attached disk. The file is the backing store. Attaching it makes the virtual disk available to the operating system; detaching it removes that access without deleting the file or its data.
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What are VHD and VHDX?
VHD and VHDX are virtual hard disk file formats, commonly identified by the .vhd and .vhdx extensions. Windows’ Disk Management workflow supports both and recommends VHDX when creating a virtual hard disk. Microsoft describes VHDX as designed to work with modern storage hardware and capabilities; the format specification overview was last updated September 30, 2022.
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The VHDX Format Specification v1.00 was published by Microsoft on July 15, 2024. The extension tells you the file format, not whether the disk is attached or how much physical storage its backing file currently occupies.
How virtual disk allocation types differ
Fixed, dynamic, and differencing describe how a virtual disk’s backing file stores data. They differ both in how storage is allocated and whether the disk relies on a parent file.
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| Type | Backing-file space | Parent dependency |
|---|---|---|
| Fixed | Allocated to the virtual disk’s full configured size. Its file size does not change as data is added or removed. | No parent required. |
| Dynamic | Grows as data is written, up to the configured maximum size. | No parent required. |
| Differencing | Stores modified blocks relative to a parent disk; reads of unchanged blocks use the parent. | Requires its parent. Without the parent, the child is not a fully functional disk. |
Fixed and dynamic disks are useful when you want a standalone virtual disk with either fully allocated backing storage or a file that grows as needed. Differencing disks are suited to keeping changes separate from a parent image, but the parent-child relationship must be preserved. Reads can travel through the child-to-parent chain, so the child file alone is not a complete substitute for that chain.
Create, attach, or detach a virtual hard disk in Windows
Windows Disk Management can create, attach, and detach VHD and VHDX files. Microsoft documents these steps for Windows versions including Windows 10 and Windows 11.
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- Create: Open Disk Management, choose Action > Create VHD, specify a file location and virtual disk size, then select the file type and virtual hard disk option. Microsoft recommends VHDX in this workflow.
- Attach: In Disk Management, choose Action > Attach VHD and select the virtual hard disk file. Once attached, the disk is available to the system.
- Detach: In Disk Management, right-click the attached disk and choose Detach VHD. The disk becomes unavailable, but detaching does not remove the backing file or erase its data.
When the distinction matters
- If you see a
.vhdor.vhdxfile, you are looking at a disk image file, not necessarily a disk currently available to the system. - If a virtual disk is detached, its contents remain in the file; attaching it again makes the disk available.
- If you have a differencing disk, keep its parent file and chain intact. The child depends on the parent for data it does not store itself.
- If you are choosing a format in Windows Disk Management, VHDX is the format Microsoft recommends in its create-disk workflow.
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