Great write up Falko. Thanks.
On testing the drives. Can you power down the system, unplug sda, power up and make sure it boots and operates properly.
Then power down again, plub sda back in, unplug sdb and boot up to make certain all is ok. If all is ok, then power down, plug in both sda & sdb, bring the system back up and let the drives sync.
Would this not work?