Sometimes when you create a RAID array using mdadm and do not specifically specify there are no spare drives available for the array, the mdadm monitoring daemon will report to you in daily emails that:
A SparesMissing event had been detected on md device /dev/md*
If you check your /etc/mdadm/mdadm.conf file, you’ll most likely find the ARRAY assembly line specifies an “spares” parameter that is non-zero.
To stop this daily nagging email message, just change the “spares” parameter to 0.
ARRAY /dev/md0 metadata=1.2 spares=0 name=<mymachine>:0 UUID=<myUUID>
The reload the monitoring daemon. In Debian:
service mdadm reload
Currently, that command will actually stop and start the daemon, rather than reloading it, but hey, who knows what the future modern world will bring?
Also, this is all assuming that you really do have no spares in your array, and the mdadm system just couldn’t believe such audacity, and concluded that you must have a least one that you just didn’t tell it about.