I have been trying to add a single mirrored disk to Open Media Vault and been getting:

In my case the new disk was created using a Debian live USB disk. I think this was the problem. So I booted to OMV and looked at the mdadm config file in Debian:

# mdadm.conf
#
# Please refer to mdadm.conf(5) for information about this file.
#

# by default, scan all partitions (/proc/partitions) for MD superblocks.
# alternatively, specify devices to scan, using wildcards if desired.
# Note, if no DEVICE line is present, then "DEVICE partitions" is assumed.
# To avoid the auto-assembly of RAID devices a pattern that CAN'T match is
# used if no RAID devices are configured.
DEVICE partitions

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# definitions of existing MD arrays
ARRAY /dev/md0 metadata=1.2 name=nas2:RAID1Mirror UUID=a77b4c17:6ddd1afc:58bf7cdd:a51275ef

# instruct the monitoring daemon where to send mail alerts
MAILADDR tony@cantabrian.co.nz
MAILFROM root

The config file can be found at:

/etc/mdadm/mdadm.conf

The problem I suspect was that the new disk was also “/dev/md0”, so I commented the line out:

ARRAY /dev/md0 metadata=1.2 name=nas2:RAID1Mirror UUID=a77b4c17:6ddd1afc:58bf7cdd:a51275ef

Then I restarted and the new disk showed up.