[ubuntu] restart after shutdown

Hi I’m newto the Linux OS, I have installed Ubuntu 15.10 on a 240GB SSD with anadditional 3TB HDD for movies/tv series, my problem is when I shutdown, thesystem restarts, I think the cause is the 3TB HDD because when I found how tospin down the HDD (after 5min not in use) the system would reboot after the5min spin down time.
The 3TB HDDis from a windows 7 OS which had my movies/tv series on.
Ubuntu pickedup the HDD with all my movies/tv shows on ok, not sure if I needed to format inUbuntu, which I did not want to do as it is nearly full.
Any helpappreciated.
Thanks.