VHD file is not a valid virtual disk file

If I use the VHD file on my local hard drive it works fine, but if I copy the VHD file to an external USB hard drive Virtual PC is not able to recognize the file as a valid virtual hard drive. I have checked file permissions, that the drive it is not read only and that there is sufficient space available. The error message displayed in Virtual PC is "The file 'filename.vhd' is not a valid virtual hard disk." and I get the error message directly when I select the VHD file under Settings in VPC. I can copy the hard drive back from the external hard drive to the local hard drive and everything works ok, so my conclusion is that there is nothing wrong with the actual file.

The host operating system is 64-bit Windows 7 Ultimate.

Any suggestions?

Best regards,
 Thomas Gustavsson