My homebrew NAS running OpenMediaVault (based on Debian) started writing to me recently. I’d had a disk failure and replaced the disk in the RAID set with another one. Everything rebuilt properly, but then this mdadm chap started sending me messages daily.

"This is an automatically generated mail message from mdadm
 running on openmediavault
A SparesMissing event had been detected on md device /dev/md0.
Faithfully yours, etc.
P.S. The /proc/mdstat file currently contains the following:
Personalities : [raid6] [raid5] [raid4] 
 md0 : active raid6 sdi[0] sda[8] sdb[6] sdc[5] sdd[4] sde[3] sdf[2] sdh[1]
 11720297472 blocks super 1.2 level 6, 512k chunk, algorithm 2 [8/8] [UUUUUUUU]
unused devices: <none>"

Which was nice of it to get in touch. But I’d never had spares configured on this md device. The fix is simple, and is outlined here and here. In short, you’ll want to edit /etc/mdadm/mdadm.conf and changes spares=1 to spares=0. This is assuming you don’t want spares configured and are relying on parity for resilience. If you do want spares configured then it’s probably best you look into the problem a little more.