Under 'Other devices', add a Floppy drive, configured to use this. zip file containing the Plop boot manager. (Historically, at least) Fusion does not allow you to boot directly from a USB drive without a third party tool such as the Plop Boot Manager, which is a small, bootable disk image allowing you to then boot from the external drive.
Of course, this may run into the same USB hurdle because that seems to be the real issue here, but it's worth trying. Since your external drive is a bootable clone, another option should be booting your VM from that external drive, and then cloning to the VMs drive rather than migrating.
