A virtual hard disk is a file that a computer or virtual machine can use as though it were a disk drive. The file is the backing store; when you attach or mount it, the operating system can access the virtual disk and its file system. Microsoft describes VHD as a format that packages a hard disk in one file for disk-like use by an operating system (Microsoft Learn: About VHD).
How a virtual hard disk works
A virtual hard disk is not a separate physical drive. It is a disk image stored as a file on an existing storage device. The image can contain a file system and data, and software presents it to the operating system or a virtual machine as disk storage.
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There are two related things to distinguish: the VHD or VHDX file is the stored image, while the virtual disk is the disk the system can use after that file has been attached. In Microsoft’s terminology, the file on storage is the backing store. Attaching it makes its contents available for disk and file operations.
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 supports both and recommends VHDX in its create-disk workflow (Microsoft Learn: Manage Virtual Hard Disks).
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Microsoft describes VHDX as a format designed to work with modern storage hardware and capabilities. The VHDX Format Specification v1.00 listing was published on July 15, 2024 (Microsoft Download Center: VHDX Format Specification v1.00).
How virtual disk allocation types differ
Fixed, dynamic, and differencing describe how a virtual disk file stores or allocates data. They differ both in how the backing file uses storage and in whether the disk relies on a parent file.
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| Type | How the backing file uses space | Parent-file dependency |
|---|---|---|
| Fixed | Allocated to the virtual disk’s full configured size. Its file size does not change as data is added or removed. | None inherent to this type. |
| Dynamic | Grows as data is written, up to the configured maximum size. The maximum capacity is not necessarily the file’s current size. | None inherent to this type. |
| Differencing | Stores blocks changed relative to a parent disk; unchanged data is read from the parent. | Requires its parent chain. The child is not a fully functional disk on its own, and reads may traverse the chain. |
Microsoft’s VHD overview covers fixed and expandable disks, while its VHDX specification describes fixed, dynamic, and differencing behavior (VHD overview; VHDX specification overview).
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Create, attach, or detach a virtual hard disk in Windows
Disk Management in Windows 10 and Windows 11 can create, attach, and detach VHD and VHDX files. The exact available actions can depend on the Windows edition and permissions. Use the Disk Management interface rather than treating the image file like a physical drive:
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- Create: Open Disk Management, select Action > Create VHD, choose a location and virtual disk size, select
.vhdor.vhdx, and choose an allocation type. - Initialize and format: After creation, initialize the new virtual disk and create a volume if Windows prompts you. Formatting prepares a file system for storing files.
- Attach: In Disk Management, select Action > Attach VHD and choose the image file. Windows then makes the virtual disk available.
- Detach: In Disk Management, right-click the virtual disk and choose Detach VHD. This makes the disk unavailable to the system but does not delete the image file or its data.
These steps and supported file types are documented in Microsoft’s Windows virtual hard disk guide.
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When the distinction matters
- If you see a
.vhdor.vhdxfile, that is the stored disk image, not necessarily a disk currently available to the operating system. - If a virtual disk is detached, its file and data remain on storage; attach the file again to make the disk available.
- If you use a differencing disk, keep its parent file and any required parent chain accessible. The child file alone does not contain all the data needed to act as a complete disk.
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