sudo dd if=/dev/zero of=/dev/sdx bs=1M count=100 conv=notrunc,noerror
repeat for each drive
Use Disk Utility
to "format drive" and give each one a new MBR.
Then "File", "Create" and have it make a RAID 5 for you:
512 stripe size
Give it a name (but do not use spaces or it screws up the mdadm.conf!)
Select the 3 drives
Then where the array size slider is, at the right is a box with the size. Round it down to the nearest 0.1GB by editing the number. Eg: If it says 2000.123GB then change it to 2000.1GB.
Then set it going.
It will give it an arbitrary device name that is not being used, /dev/mdN
Click on the RAID device icon on the left and then the partition in the graphic and then "Format Volume".
Finally, add the ARRAY directive to mdadm.conf.
copy the line for the new RAID to mdadm.conf and change the device name to /dev/md? as you wish.
Delete or comment out the old RAID5 one.
Stop the array and reassemble it to make sure mdadm.conf is right
mdadm --stop /dev/mdN (its current name)
check it reports assembling it with the device name you specified in mdadm.conf
edit fstab to mount it at boot with its new device name and then
and check it is correctly mounted
Once all is ok,
sudo update-initramfs -u