What’s a Virtual Hard Disk?

A virtual hard disk is a “fake” hard disk.  It’s usually just a file (or a list of files in a folder) that is treated as if it were an entire hard disk when using Virtual Machine software such as VMWare, Virtual PC, or Xen.  All of these products are programs that you can run, just like any other program on your computer, but instead of opening a document, like a spreadsheet, image, or text file, they open a virtual hard disk.  In fact, the program window displays what looks like your machine booting up from a complete power off state, showing the BIOS setup options, memory checks, the operating system booting up, until it eventually gets to the desktop.  But, it does all this, in a window!

For example, below, is screen shot of my Windows 7 Ultimate desktop, with FireFox open to my blog, and in the middle of the screen, a Virtual Machine that’s been booted up to the Ubuntu 8.10 Linux desktop.  Inside of that virtual machine, is the Linux version of FireFox open to a web page describing Ubuntu.

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Now, my real boot drive does NOT have Linux on it.  It boots directly to Windows 7.  So the question is how does the virtual machine boot up to anything different?  Instead of booting up on a real hard drive, when I set up the Linux virtual machine, I told it to use a file instead of a real hard drive.  From my Windows 7 host machine, it’s just a file (a large file, but just a file, buried in a folder of my choosing).  When the virtual machine goes through its simulated boot cycle, the virtual machine software (VMWare in this case) tricks the VM (virtual machine) into thinking that the file is an actual hard drive and Linux happily uses it as such.  That file that’s on my Windows 7 machine, is the “Virtual Hard Disk”.

Windows 7 has a new feature that let’s you really boot from a virtual hard drive file.  But, it has to be a Virtual PC compatible hard drive file.  I couldn’t do it with my VMWare compatible virtual hard drive file that I created for the ubuntu Linux VM in the image above.

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