One of the joys of virtualisation is the flexibility afforded by the ability to copy virtual machine files around the network for backup purposes or just to create a new machine (especially with Microsoft’s new Virtual Server licensing arrangements). Unfortunately, just as for “real” computers, simple file copies of Windows-based virtual machines can cause problems and are not supported (see Microsoft knowledge base article 162001).

All is not lost though, as Microsoft does support the duplication of virtual hard disks using the system preparation tool (SysPrep) and Megan Davis has written about sysprepping virtual machines on her blog. I tested it today and it works really well – basically a 3 step process of:

1. Install and configure a source virtual machine as required (i.e. operating system installed, virtual machine additions installed, service packs and other updates applied), making sure it is in a workgroup (i.e. not a domain member).
2. Locate the appropriate version of the Windows deployment tools (I used the ones from the \support\tools\deploy.cab file on a Windows Server 2003 CD) and create an answer file (C:\sysprep\sysprep.inf). Then copy the sysprep.exe and setupcl.exe deployment tools to C:\sysprep.
3. Run SysPrep to reseal and shut down the guest operating system, then copy the virtualmachinename.vhd file to a secure location (make it read-only to prevent accidental overwrites, but also apply appropriate NTFS permissions). This file can then be duplicated at will to quickly create new virtual machines with a fully-configured operating system.

For anyone who is unfamiliar with SysPrep, check out Killan’s guide to SysPrep (which, despite claiming not to be written for corporate administrators or OEM system builders, seems like a pretty good reference to me).

Incidentally, there are major performance gains to be had by moving virtual machines onto another disk (spindle – not just partition). Unfortunately my repurposed laptop hard disks were too slow (especially on a USB 1.1 connection), so I had to go out this afternoon and buy a USB 2.0 PCI adapter along with a decent external hard disk (a Toshiba 320GB 7200 RPM external USB 2.0 hard drive with 8MB data buffer) – that speeded things up nicely.