Impossible to stop raid device

/dev/md1 is the RAID array itself. You need to stop a particular participant in that array. Try:

mdadm --fail /dev/md0 /dev/sda1
mdadm --remove /dev/md0 /dev/sda1

This is a good resource: http://www.excaliburtech.net/archives/19

If you need to move all the participating devices, then you need to do this one by one, waiting for the array to rebuild after each device swap.