Nodachi swords are the swords that were carried on the back for the field battle. The length of the sword made it suitable to be used only on the open battlefield. They were not meant for close or indoor combats. The nodachi and odachi could not be worn on the waist. The term nodachi was mostly sued to describe the larger Japanese swords.
Later the two terms nodachi and odachi were used for the long swords. The nodachi was the long sword that was used by the samurais before the discovery of the smaller swords. The long swords were similar in design and style to the modern day katana sword. The difference lies in the length of the sword and the way it was worn by the samurai.
The katana sword was discovered as it was not feasible for the warriors to use a long sword for the indoor combats. It was also difficult to draw out the long sword from the back as it was quite heavy and long. Later, another man used to walk behind the swordsman who could draw out the nodachi sword for the warrior. The nodachi and odachi swords were used for making strong striking actions and killing the cavalryman and his horse in a single action. It required great strength and power to use the nodachi sword effectively.
The nodachi and odachi swords were used by the foot soldiers as it was not possible by the cavalrymen to carry the long swords on their back while riding the horse. It was very difficult to make the long swords as it required greater skill to make the long swords. It also took many days to make a single nodachi sword. Only the skilled and experienced sword smiths were appointed for making the long swords. It was difficult to give the heat treatment to such swords. The sword smiths would hang down the swords through a long cord to give the heat treatment. Thus, if you find a nodachi or an odachi sword, it is the same sword and there is not much difference.