Result: it works now - one of the disks was faulty. I run raid creation once more - with no modifications at all. Instead of kernel bug there were messages that disk (sdd) is faulty (98% => after 3 hours). I removed it and everything works now (no spare disk at the moment). So I think that in kernel is some hazard in case of faulty hardware. But it can be complicated to reproduce, since one can not know, what exactly went wrong. -- Sincerely Ivo Panacek