Mac, sure you can only have one "active" partition on a given drive, but since we are talking about marking a partition as active on the HD that is in the caddy it seems to work OK.
As reported on your "Dodge" thread, I failed to do this the first time and had to repeat the rigmarole of putting the new drive back into the caddy, and reworking it with Partition Magic to make the new "cloned" partition active.
Then when I put it back into the Sony Vaio as the only drive the laptop had no choice but to boot from it.
I don't think this will work with Fdisk. Fdisk only sees disks that are on the IDE or SCSI interfaces. I think you have to use a Partition Manager like Partition Magic on drives that are just using data pipes like Firewire or USB.