For my knowledge chewitt unit file looks absolutely the right way, but i spotted a letter missing. He does not run my image and could not check and see that mdadm is in /usr/sbin/mdadm not /usr/bin/mdadm.
So take chewitt unit file and correct the path of mdadm should do the job.
Did it work after you corrected the path?
I mean, chewitt said that mdadm should also be in that path.
Please test that.
Good luck!
P.S.:
ExecStart=/usr/bin/mount /dev/md0 /media/RAID
That´ s right?
Does "systemctl status raid5.service" show any errors from the commands when they are run? - Are there any errors in the systemd journal or showing in dmesg? I'm wondering whether some sleep delays might be required space out commands? - filesystem tasks can need time to complete/settle before you run the next command; systemd ExecStart sequences need the same considerations as putting commands in a bash script.
After I fixed the error in your file (using `use` to `usr`) and the error in `bin/mdadm` to `sbin/mdadm`, which sky42 pointed out, it actually works now 
God bless the king!
Many thanks to both of you for the great help.
