Make USB hard disk with OS X bootable as startup disk? Some years ago it was easy to clone the current system partition of a Mac computer to an external firewire hard disk and then use this as a startup disk for the same Mac or even for another Mac.

When I currently tried this with a USB drive (Western Digital Expansion+) and cloned the SSD of my MacBook to it and tried to boot another MacBook (different model) with it (by holding option key while booting), it was not recognized as a possible startup disk.

What could be the reason for it? Are USB drives more difficult than Firewire as startup disks? Or is it more difficult today to use the same system configuration on a different MacBook model?

The source was the SSD of a MacBook mid 2012. I'm not sure and currently can not find out, how old the other MacBook was...
Recently I ran into a similar problem. See 
Problem cloning the startup disk of a MacBook Pro running OS X 10.11.6 El Capitan .

Keep an eye on how you cloned your system partition residing on the SSD. I guess your system partition is of type  CoreStorage. If so, the recipe I figured out for cloning the startup disk may be useful to you.