My version of Virtual PC is 2007 SP1, and the .exe has version number 6.0.192.0
You could try upgrading to SP1.
http://www.microsoft.com/downloads/d...7-f6c7a1f000b5
(this presumes there are variations in vhd between different versions - no idea how likely that is though)
Also try running disk management inside your virtual machine (start, run, diskmgmt.msc) and see if that "drive" is detected.
Just to try, try loading only 2 virtual hard disks (1 and 3) and see how it goes (load 3 as second virtual hard disk).