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A virtual hard drive is a file that a computer or virtual machine presents as a disk. It can hold a file system and be used for ordinary disk and file operations, but it is not a separate physical drive—and it is not a complete virtual machine.
What a virtual hard drive is
Microsoft describes a virtual hard disk (VHD) as a disk image that encapsulates a hard disk in a single file for use by an operating system as a virtual disk. In practice, the file stands in for disk storage: software presents it as a disk, and the operating system can work with the file systems and data inside it. Microsoft’s Windows management guide covers VHDs, while its VHD overview explains the format.
The image file is only the disk component. A virtual machine also needs a virtualization program, configuration, and other virtual devices. Mounting a disk image makes its storage available; it does not start a virtual machine.
How a virtual hard drive works
The host computer stores the image file on physical storage, such as an internal drive. The operating system or virtualization software presents that image as a disk to the host or to a guest operating system. In VirtualBox, for example, guest read and write requests are redirected to the disk image file, according to Oracle’s VirtualBox storage documentation.
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A VHD can contain a file system such as NTFS, FAT, exFAT, or UDFS, as Microsoft’s VHD overview describes. To the system using it, the image provides disk storage; on the host, it remains a file.
VHD, VHDX, VDI, and VMDK formats
These extensions identify disk-image formats. Which formats can be created or used depends on the operating system and virtualization software; they are not guaranteed to be interchangeable across products.
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| Format | What it means |
|---|---|
| VHD (.vhd) | Microsoft’s virtual hard disk image format. |
| VHDX (.vhdx) | Microsoft’s VHD v2 format, designed for modern storage configurations. Microsoft’s Windows management guide recommends VHDX for its documented workflow. |
| VDI | VirtualBox’s native disk-image format. |
| VMDK | A disk-image format supported by VirtualBox, alongside VHD and VDI, according to Oracle’s documentation. |
Check the documentation for the specific hypervisor or operating system before moving an image between products. Support for one format does not establish that every product can use it or preserve all of its behavior.
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Virtual disk allocation affects the size of the host file and how changes are stored. Microsoft’s VHD management documentation and VHDX overview describe fixed, dynamic, and differencing types.
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Fixed
A fixed virtual disk allocates a host file to the full configured virtual-disk size. That space is reserved even if the disk contains less data.
Dynamic
A dynamically expanding disk starts as a smaller host file and grows as data is written, up to its configured virtual capacity. Its current file size and the capacity presented to the operating system using it are different measurements.
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Differencing
A differencing disk stores changed blocks relative to a parent image. Accessing data may involve the child image and its parent chain, so the child depends on the parent. Keep the whole chain together when moving or backing up the disk; copying only the child does not preserve a self-contained disk.
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Create, attach, or detach a VHD in Windows
Microsoft’s Disk Management instructions apply to Windows 10 and Windows 11 and the Windows Server releases listed in its guide. The documented workflow requires administrative privileges. A newly created VHD is initialized as part of the process.
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- Create: Open Disk Management with administrator privileges, then select Action > Create VHD. Choose the file location and name, set the virtual hard disk size, and select a fixed or dynamically expanding type. Follow the prompts to create and initialize it.
- Attach an existing file: In Disk Management, select Action > Attach VHD, browse to the VHD or VHDX file, and confirm. Windows makes the disk available for use.
- Detach when finished: In Disk Management, locate the attached virtual disk, open its context menu, and choose Detach VHD. Detaching makes the disk unavailable but does not delete the image file or its data, as Microsoft explains.
Use the detach operation when you want to stop using the disk without removing its stored contents. Deleting the backing file is a separate action and can destroy the data it contains.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What VHDX improves—and what its capacity figures mean
Microsoft describes VHDX as optimized for modern storage hardware. A separate Microsoft overview for Windows Server 2012 and Windows Server 2012 R2 states that VHDX supports virtual disks of up to 64 TB and describes metadata logging, improved alignment for large-sector disks, and trim-related efficiency. That 64 TB figure belongs to that historical, platform-specific documentation; it should not be treated as a universal current limit for every product or deployment. See Microsoft’s VHDX overview and its Windows Server 2012/2012 R2 Hyper-V format overview.
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