Mateusz [off-list ref]

Should in most cases [1], but IMHO it's good idea to
mdadm --zero-superblock /dev/YOU_ARE_SURE_IS_ONE_YOU_WANT_TO_ADD
before adding disk already used somewhere else.

   "madam --zero-superblock" is great.  I will add it to my procedure.
   thanks a lot for the hint!

BTW,  IMHO it's better to clone partition layout, and than install bootloaders
instead of dding disk.

   yes. but sometimes dd is easy, especially for mbr layout.
anyway I think your suggestion "--zero-superblock" make things safe
with new and old disks.

   thanks again for your kindly help.