The short answer
- VHD is a disk-image format that encapsulates a hard disk in one file and exposes it to an operating system like a physical disk, with dynamic and differencing disks limited to 2,040 GB.
- On Windows 10, Windows 11, and supported Windows Server releases, use Disk Management to attach the .vhd file and detach it when finished. PowerShell mounting requires administrator privileges. For an image from an unknown source, choose read-only attachment when available and inspect its files without running them. Microsoft documents attaching VHD files on Windows only; the sources here do not cover attaching a VHD directly on macOS, Android, or iOS.
- A VHD is not inherently executable, but it may contain applications or a bootable operating system. Attaching it exposes its volumes like an ordinary disk. Treat unknown images as untrusted, prefer read-only attachment, and do not execute their contents. A valid footer identifies the container structure; it does not establish that the contained files are trustworthy.
VHD file: identify, open, inspect, and convert a Virtual Hard Disk
Microsoft's publicly documented Virtual Hard Disk format can hold partitions, NTFS, FAT, exFAT, or UDFS file systems, applications, and a bootable operating system. It supports fixed, dynamically expanding, and differencing disks. Every conforming image has a 512-byte footer containing an eight-byte "conectix" cookie and other metadata.
Convert-VHD copies a detached source disk into a new VHD or VHDX offline and can change the container format, allocation type, and block size. Never convert or modify an image used by a running virtual machine or another process. For a differencing disk, retain and supply the complete parent chain and correct parent path.
How to open it
On Windows 10, Windows 11, and supported Windows Server releases, use Disk Management to attach the .vhd file and detach it when finished. PowerShell mounting requires administrator privileges. For an image from an unknown source, choose read-only attachment when available and inspect its files without running them. Microsoft documents attaching VHD files on Windows only; the sources here do not cover attaching a VHD directly on macOS, Android, or iOS.
On Windows 10, Windows 11, and supported Windows Server releases, use Disk Management to attach the .vhd file and detach it when finished. PowerShell mounting requires administrator privileges. For an image from an unknown source, choose read-only attachment when available and inspect its files without running them. Microsoft documents attaching VHD files on Windows only; the sources here do not cover attaching a VHD directly on macOS, Android, or iOS.
Convert-VHD copies a detached source disk into a new VHD or VHDX offline and can change the container format, allocation type, and block size. Never convert or modify an image used by a running virtual machine or another process. For a differencing disk, retain and supply the complete parent chain and correct parent path.
File safety
A VHD is not inherently executable, but it may contain applications or a bootable operating system. Attaching it exposes its volumes like an ordinary disk. Treat unknown images as untrusted, prefer read-only attachment, and do not execute their contents. A valid footer identifies the container structure; it does not establish that the contained files are trustworthy.
An unknown sender, a name ending in two extensions, or contents that do not match the extension — check before you open it.
Converting it
Convert-VHD copies a detached source disk into a new VHD or VHDX offline and can change the container format, allocation type, and block size. Never convert or modify an image used by a running virtual machine or another process. For a differencing disk, retain and supply the complete parent chain and correct parent path.
Convert a copy, never the original, then compare quality, metadata, and layout against what you started with.
Common questions
What is a VHD file and what is it used for?
A VHD is a Microsoft disk-image container that represents a hard disk in one file. It can contain partitions, native file systems, applications, or a bootable operating system and can act as a virtual machine's boot disk.
How do I open a VHD file?
Attach the VHD with Disk Management on Windows 10, Windows 11, or a supported Windows Server release. Its volumes then become available like those of an ordinary disk; detach the image when finished.
How do I mount a VHD file in Windows 11?
Open Disk Management and use its VHD attachment action, select the .vhd file, and attach it. For an unknown image, use read-only attachment when available. PowerShell mounting requires administrator privileges.
Why will my VHD file not open or attach?
Check that the file is not in use by a running virtual machine or another process. For a differencing VHD, also verify that its unchanged parent image and the complete parent chain remain available at the correct paths.
Should I use VHD or VHDX for current Windows disk management?
Microsoft recommends the newer VHDX type for current Windows disk-management workflows. VHD remains the documented older format and has a 2,040 GB maximum for dynamic or differencing virtual disks.
How do I convert VHD to VHDX without damaging the image?
Detach the source first, then use Convert-VHD to make a new VHDX offline. Do not convert an image used by a running virtual machine or another process. Preserve the full parent chain for a differencing disk.
Is a VHD file from an unknown source safe to attach?
Do not assume so. Although VHD is not inherently executable, it can contain programs or a bootable operating system. Prefer read-only attachment and inspect the files without executing them.
What is the maximum size of a dynamic or differencing VHD file?
The supported maximum virtual-disk size for a dynamic or differencing VHD is 2,040 GB.
How does a VHD differencing disk use its parent path?
It stores subsequent writes separately while relying on an unchanged parent image. Conversion requires the correct parent path, so retain and supply the entire parent chain.
Does a VHD conectix footer signature prove the file is trustworthy?
No. A conforming VHD has the ASCII "conectix" cookie in its 512-byte footer, but that structural identifier does not establish that files or software inside the image are trustworthy.
Related formats
Sources
VHD Specifications · Microsoft Download Center — official format specification
About VHD · Microsoft Learn — official Windows Virtual Storage documentation
Manage Virtual Hard Disks (VHDs) · Microsoft Learn — official Windows Server documentation
Convert-VHD · Microsoft Learn — official Hyper-V PowerShell documentation
Mount-DiskImage · Microsoft Learn — official PowerShell Storage documentation
IVdsOpenVDisk::Attach · Microsoft Learn — official Windows API documentation
QEMU disk image utility · QEMU Project — official documentation