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These articles provide basic information and helpful recommendations concerning data access and recovery.

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Virtualization software: virtual disks

In recent years virtualization technology has managed to gain wide popularity. It is based on the idea of creating a fully operational isolated system within the main system on the basis of the resources of the same hardware.

Nowadays virtual machines are available not only for corporate, but also for home use. They enable users to leverage hardware resources and are quite reliable when utilized for backup purposes, allowing the “home” system to remain unaffected in case of failures.

What is a virtual disk?

virtual machine is special software that emulates the operation of a physical machine. A virtual machine is capable of running its own operating system and applications independently from the physical one.

Based on the resources of a physical machine, a virtual machine remains completely independent and uses its own software-based components (the CPU, motherboard, video adapter, network adapters and hard disks), which may differ from those of the “real” machine it is installed on. However, a virtual machine is unable to go beyond the limits of its virtuality and should be located within a real physical machine and make use of its resources. The operating system the virtual machine is installed on is called the host OS, while the operating system of the virtual machine is referred to as the guest OS.

virtual disk is a file located on a host OS with all information about the data of a guest OS and is used to isolate a virtual machine from the physical one. More than one virtual disk can be operated simultaneously on a single physical drive, each separated from the another. A virtual disk may be a part of a virtual machine or may be mounted on the OS of a physical machine.

How can the data be accessed?

A usual way to exchange information between a host OS and a guest OS is to run both of them and use a virtual network transport. Virtualization software often offers transport wrapper software for a guest OS, allowing file exchange with the help of a simple drag-and-drop procedure.

However, data exchange may require much time or simply be impossible in the following situations:

  • Getting data from a historical virtual machine snapshot: – running a virtual machine is not recommended in order to avoid modifying the snapshot. It's required to copy the virtual disk with the setup and boot of a new virtual machine.
  • Utilities of the virtual machine are not installed for some reason:
    – either because of isolation of the virtual machine or non-availability of utilities for a guest OS.
  • No special networking protocols are installed on the guest OS: – isolation of the virtual machine does not allow file transfer.
  • File size limit:
    – the software may have problems with copying very large files from the guest OS to the host OS.

Yet, there is a better solution for files exchange between the guest and the host operating systems. As required files are already stored on the host computer inside the virtual disk, it's possible just to extract these files from the virtual disk at a logical level.

We offer UFS Explorer Standard Access as a perfect solution for opening such virtual disk files, browsing files and folders on them and copying files out of a virtual disk to the host OS storage. In case of data loss from a virtual machine, our data recovery products help to get your data back with maximum possible result. All products support both flat and sparse virtual disks as well as virtual disk formats of major virtualization software vendors.


Some virtualization products implement specific techniques for virtual disks related to optimization and compatibility with a host OS which make data extraction from virtual disks more complicated. Among them are:

Chunked virtual disks introduced by VMware to create FAT-compatible virtual disks. Since FAT has limitations to file size, VMware products support splitting virtual disk to fragments (chunks) that are not larger than 2 GB each. To read such chunked virtual disks it is required to assemble the chunks into one virtual image. The function is available in UFS Explorer Professional Recovery and UFS Explorer RAID Recovery through tools for RAID-building or via RAID Access Plugin - RAID Builder for all standard UFS Explorer software.

Virtual machine package introduced by Parallels for their Paralles Desktop for Mac product. A virtual disk is packed into a single file along with its snapshots. To get data from this kind of a virtual disk one must extract it from the package first.

ESX Server Storage virtual disks can be shared over the network from VMware ESX Server, however, there is a known issue of file sharing software for ESX that virtual disk files are often shared as blank. To access a file from such a virtual disk, the file should be transferred from ESX Server to the 'local' storage first.

Last update: 20.06.2018