I tried to get the mountpoint with the command from @tony but the result always was "The uuid you supplied was invalid." To get a new/live log i restarted the toolstack (the 100th time..) and now everything is working again. I dont know why but okay.. Thank you very, very much @olivierlambert and @tony for your help!!!!

In other words, you must empty the host before add it to the primary pool, of course, you can migrate VM from one pool to another. And if your hw is not the same, but similar, you'll have an heterogeneous pool.

Just nothing right now. VMs will continue to run. You won't be able to send orders (like moving VM, stop, or start new VMs). If your master is down for good, you can emergency transition the master to a slave (but you must be SURE the previous master won't come back!).