VM Workstation V9 "the file specified is not a virtual disk"

I am using Workstation 9 and run all of my VM's from an external hard drive.   When I open up two of my most frequently used VM's the "Hard Disk" under devices is blank and when I try to start the VM it gives me the error "the file specified is not a virtual disk"

It looks like all of my .vmdk's are present and accounted for.

Any ideas why the "Hard Disk" field is blank?

Thank you for any ideas.