mdadm may sometimes send a "degraded array alert" reporting that at least one array is in auto-readonly
md1 : active (auto-read-only) raid1 sdc1[1] sdd1[0]
This most commonly happens after a restart/power-event and isn't usually an issue - MD arrays will be auto-read-only until they're first written too. It happens to try and help make array assembly a bit safer - nothing's written to disk until it actually needs to be
# Assuming md1 is the array in auto-read-only
# Look for md1 to find which mount point is used for the array
lsblk | less
# If a mount point was found, verify it's mounted (replace $MOUNTPOINT
# with the path) and cd there
mountpoint $MOUNTPOINT
cd $MOUNTPOINT
# Write to disk
touch test
rm test
# Verify that the array is no longer auto-read-only
cat /proc/mdstat
# If no mountpoint was found, then you *can* use mdadm to make the array readwrite
# though there isn't really any need
mdadm --readwrite /dev/md1
cat /proc/mdstat | head -n 4 # show only the first 4 lines
Personalities : [raid1]
md5 : active (auto-read-only) raid1 sde[0] sdb[2]
3906887488 blocks super 1.2 [2/2] [UU]
bitmap: 0/30 pages [0KB], 65536KB chunk
# Find a mount point underpinned by that array
lsblk | less
md5 9:5 0 3.7T 0 raid1
VolGroupRaid-lvmusic 253:10 0 320G 0 lvm /mnt/music
# Verify it's mounted
mountpoint /mnt/music
/mnt/music/ is a mountpoint
# cd into it and write a file
cd /mnt/music
touch test
rm test
# Verify it's now read-write
cat /proc/mdstat | head -n4
Personalities : [raid1]
md5 : active raid1 sde[0] sdb[2]
3906887488 blocks super 1.2 [2/2] [UU]
bitmap: 0/30 pages [0KB], 65536KB chunk