Software raid is really great in Linux, but once you need to repurpose old drives,
or lets say some other implementation of software raid has fucked up your disks,
you need to do some clean up.

This basically has to happen in two steps so it can succeed:

• stop the md device
• remove the super block from the device

So here you go, to find out which md device is active run this command:

cat /proc/mdstat

 in the list figure out which md device you want to delete / stop. Then runs this command:

mdadm --stop /dev/md127

 After that succeeded the last thing you need to make care of is the super block:

mdadm --zero-superblock /dev/sda1

Devices paths are of course just examples, make sure you double check on this, otherwise you might loose data!