Bootable backup - Mac


I have had for years a Drobo 2nd Gen directly attached to a Mac as a bootable back created by Superduper.
A few months ago I bought a 5C, directly attached and created bootable backups with SD - all on High Sierra now.
For testing purposes I tried to boot but it does not work.
Wit Power+Option I only get the HD as an option but not the Drobo.

I can select the Drobo in preferences as the startup disc but it boots nevertheless from HD

What is going on?