I replaced my Superdrive with a second hard drive, and now I've got spin-up/down issues
Run sudo pmset disksleep 0, with an optional -a, -b, -cor -u between the pmset and the disksleep.

See man pmset for details:


  The -a, -b, -c, -u flags determine whether the settings apply to battery ( -b ), charger (wall power) ( -c ), UPS ( -u ) or all ( -a ).