Essa é uma pergunta antiga, mas, como pesquisei por uma solução bastante longa, quero compartilhar meu resultado:
# /etc/mdadm/mdadm.conf
ARRAY <ignore> UUID=3f620e6d:4e655d66:b931eb71:baf7cf3a
De man mdadm.conf
:
ARRAY The ARRAY lines identify actual arrays. The second word on the line may be the name of the device where the array
is normally assembled, such as /dev/md1 or /dev/md/backup. If the name does not start with a slash ('/'), it is
treated as being in /dev/md/. Alternately the word <ignore> (complete with angle brackets) can be given in which
case any array which matches the rest of the line will never be automatically assembled. If no device name is
given, mdadm will use various heuristics to determine an appropriate name.