How to delete an mdadm RAID Array
Deleting a software RAID array will result in all data stored in the devices being lost. So be careful when you remove a RAID array, take backups in case needed.
Before you can remove the software RAID array, you need to unmount it.
umount /dev/mdX
Where /dev/mdX is the device name for the RAID device you need to remove.
Find the disk used to create the RAID with the command
mdadm --detail /dev/mdX
Stop the RAID device with the command
mdadm --stop /dev/mdX
Now you need to run the following commands for each storage device that are part of the RAID device.
mdadm --zero-superblock /dev/sdXY
IMPORTANT: you need to run this for each member of the RAID device.
Finally, edit /etc/mdadm/mdadm.conf, and remove the entry for the RAID array.