The Mega Drive was renamed Genesis for its 1989 American launch.
Lack of awareness of the TurboGrafx-16 left the NES as the Genesis' biggest competitor until the release of the SNES. By this time the Genesis had established a large user base and was able to outsell the SNES in America. Shortly after the release of Sonic The Hedgehog 2 (on Sonic 2's Day, which as you probably guessed was a Tuesday!) the Genesis held a massive 65% share of the market.
However, the success of the console in the States, and as the Mega Drive in Europe, may well have contributed to Sega's downfall. Two misjudged pieces of hardware, the 32X and Sega CD put a huge dent in Sega's reputation, one so large that the company would never recover.
A redesigned Genesis II was released in 1994. The smaller console was cheaper to produce, similar in concept to the Master System II.
Taneli Lukka from Filand adds :
Connecting the 32X addon to the original Genesis/Mega Drive is very difficult because a special adapter cable is required to link the 32X unit and the MD console together. This happens because MD1 has a diffirent AV port than the MD2. This cable was very hard to get even when 32X was released (many people had to buy a MD2 unit to get their 32X working) and it's almost impossible to get your hands on one of these cables today.
James (UK) comments :
I believe i have a seen two different versions which havent been mentioned here. the Mega Drive i own has a white piece of plastic around the power L.E.D, and no writing at all on the ring. it has no EXT port and all plastic feet and only mono sound from the a.v out port - i modified it for stereo by taking the output from the headphone port. The Mega Drive my uncle had was one of the first in the UK. That had a GREY piece of plastic around the led and had 'high definition audio' only written on the ring :)