is the array you want to delete, this command will stop the array and release /dev/sdc
mdadm --misc --stop /dev/md126
To make sure the md
driver doesn't recreate the unwanted array, delete the superblock on /dev/sdc
mdadm --misc --zero-superblock /dev/sdc
Depending on how the distribution is configured, you may have to run these commands via sudo
You should then be able to add /dev/sdc
to the remaining array from the GUI.