Mac Forum / Applications / Virtual PC / December 2005

So I'll confess I use differencing disks more on Virtual Server.  As
you've seen, the concept in VPC 2004 is that you create a vhd that
points to a base vhd.  The interesting thing is that Undo disks are
basically differencing disks with a different extension (vud v. vhd).
Even without a "create diff disk" utility you should be able to rename a
vud to a differencing disk.  That's the theory.

On the Mac, a virtual hard disk is not only a file, it's a directory
with these files:

virtualdisk.vhdp:  (Directory)
   MainPackageFile
   BaseDrive.vhd
and if undo drives are enabled,
   UndoDrive1.vud

The problem with the above is the hardcoded name for the BaseDrive. I
tried to use a symbolic link to make this hierarchy:
virtualdisk.vhdp:
   MainPackageFile
   BaseDrive.vhd -> diff/virtualdisk-diff.vhd
   /diff:
       BaseDrive.vhd
       virtualdisk-diff.vhd (the renamed Undo drive)

The reason for this hierarchy is that an Undo has a filename reference
to the Parent disk, so if the parent is named "BaseDrive.vhd" you have
to preserve that name.  The Parent's name can be edited in the
Differencing Disk but that requires changing it in up to three places
and updating a checksum all in a proprietary binary format.

The problem with the above is that VPC doesn't honor symlinks for a VHD
file.  :(  It somehow thinks the file is in use in another machine.  I
think it's just confused by the link.

So...this brings me back to square 1.  VPC Mac is capable of running VPC
2004 machines.  So if you create a differencing disk chain there, I'm
pretty sure VPC Mac will honor it.  As you've seen, I let my theory get
ahead of reality.  If you can try your own experiments, I would be
interested in the results.

BTW, if you're unaware of this.  To see the BaseDrive.vhd file in the
Finder you must select the virtual hard disk icon and choose "Show
Package Contents" in the context menu.

Good luck,
Richard