If you have NTFS on the partition it could be Windows bugged up the cleaness of the system. Reboot into windows and shutdown cleanly or fix it from Linux

Commands

smartmontools package to check your disk’s
smartctl -a /dev/sda

cat /proc/mdstat

# Your md config
cat /etc/mdadm/mdadm.conf

# These works but does not solve the problem
sudo umount /media/jsf/RAID5
mdadm –stop /dev/md126
sudo mdadm –detail /dev/md126
sudo mdadm –assemble –scan

# To make them read write, but does not work, still read-only
mdadm –readwrite /dev/md126

Refs:
https://bbs.archlinux.org/viewtopic.php?id=137058
https://unix.stackexchange.com/questions/402771/raid1-is-read-only-after-upgrade-to-ubuntu-17-10