Today I started up Windows 7 on my iMac. Then I tried to start up Windows XP but got this error.
Failed to open a session for the virtual machine Windows XP Pro Media Center.
PIIX3 cannot attach drive to the Secondary Master
Unknown error creating VM (VERR_SHARING_VIOLATION).
Fortunately Google came to the rescue and led me to this article in the VirtualBox forums, Can’t run multiple Instances on OSX Leapord. Once I unmounted the DVD drive in Windows 7 (Devices –> CD/DVD Devices) I was able to start Windows XP.
The lesson is you can’t run two instances if both want to mount the same DVD drive. Not sure if this is a problem with Parallels or VMWare.