No mdadm --detail only shows the one that I can see in OMV. I am using OMV 5 because had issues with OMV 6 on the supermicro hardware I am using.. 5 seems to work better.
I put my OMV4 drive back in and I see this in my mdadm.conf
# definitions of existing MD arrays
ARRAY /dev/md/OMV-Archive:Archive4tb metadata=1.2 name=OMV-Archive:Archive4tb UUID=600f433a:1e0bc059:907cd93f:4a7164aa
ARRAY /dev/md/OMV-Archive:Archive6tb metadata=1.2 name=OMV-Archive:Archive6tb UUID=67a427ef:253143bf:df315f9d:ff426d60
ARRAY /dev/md0 metadata=1.2 spares=2 name=OMV-Archive:Archive12tb UUID=4d5183f8:8aabc3fa:63e60eb4:46772e82
But only the 4tb is started and when I examine the drives for the others I get the No md superblock detected..