I noticed that there's a couple of threads floating around where people are asking about whether they can get imported games to work, and how to do so. I've typed up my thoughts on this, so hopefully we can put to bed the importing issue once and for all. If you know anything worth adding, or notice any errors, by all means post it and I'll edit this!
- The TMSS (Trade Mark Security System) - i.e. the 'PRODUCED BY OR UNDER LICENSE FROM SEGA ENTERPRISES LTD.' screen - does not affect your ability to import and play foreign games. Its origins are inspected in Melf's 2007 article, but for now be certain that whether your system has the TMSS or not, it is irrelevant if you wish to play imports. (For the record, its purpose is to prevent unlicensed game development - not importing.)
- The region lock is entirely determined by the game itself, and not by your console. Put simply, the Mega Drive has two set values in its memory that indicate where it is from. If it is region-locked, the game reads these memory values and decides whether or not to start up. So, to get around this lock, all you have to do is modify these values. There are a number of ways to do this, each with varying levels of effort and expense. These are covered by the various methods below.
WILL [GAME] WORK ON MY CONSOLE?
In a nutshell: possibly, depending on the game. There are a couple of lists of region-locked games maintained elsewhere on the Web - however, there are 'rules of thumb' which can help in a pinch:
- Check the date. On the back of the box there will be a copyright (c) mark, followed by a year - my copy of Space Harrier II, for example, reads (c) 1990 SEGA ENTERPRISES LTD. Now the region lock was introduced at the beginning of 1993, so generally any game released during or later than 1993 will be region-locked.
- Check the publisher. EA games used the exact same cartridge style worldwide (even mentioning the Genesis in PAL territories), and they are never region-locked no matter when they were released. So, if the box displays the Electronic Arts logo, and/or the cartridge has a yellow tab on the left-hand side, the game is not region-locked.
- Check the ROM. Whilst nobody at Sega-16 condones illegal emulation, it's a trivial matter to fire up your emulator of choice, set it to a certain region, and then open up a particular game ROM file to see if it works.
- Check the Internet. Generally, a quick search of the Sega-16 forums won't be as immediate and as effortless as posting a new thread and being spoonfed information, but if it's been asked before it's also probably been answered before.
HOW DO I GET IT TO WORK?
So, you've verified that the game is region-locked, but you'd still like to play it on real hardware. Here's how.
- METHOD 1 - Imported game, imported console.
(Skill - nil, Effort - low, Expense - medium (variable), Reliability - guaranteed)
This isn't really a technical solution, but it means you're pretty much guaranteed to run imports just fine. If you have a serious aversion to either picking up a soldering iron or piercing the skin of your consoles, this is probably your safest option. On the other hand, you'll need to work out a way of getting the foreign machine to play nice with your local power supply and TV equipment. Plus, you'll obviously need to purchase another machine, meaning you'll need a) money and b) space.
- METHOD 2 - Cartridge slot extender
(Skill - nil, Effort - nil, Expense - minimal, Reliability - somewhat)
These devices are shaped a little like the Sonic & Knuckles cartridge, but don't do any 'magic' of their own - their sole purpose is to extend the cartridge slot of the main console, so that the cartridge shape difference doesn't matter. Japanese games won't fit inside an American or European machine, so you plug a Japanese cartridge into the top and plug the combined units into your console. These are generally dirt-cheap on your auction site of choice. Beware, however, sellers who advertise these as the end-all solution to importing - all these converters to is make it easier for you to fit foreign games into your console. All these are really good for is getting pre-1993 Japanese games to fit into your machine.
Similar cartridges do exist that offer DIP switches that you set to the appropriate country - these are reported to be unreliable and fragile, so unless you're getting a good bargain price for one I wouldn't bother.
- METHOD 2a - Console Circumcision
(Skill - minimal, Effort - minimal, Expense - minimum, Reliability - somewhat)
The first region lock put into place by SEGA was to change the shape of the cartridge slot when it was first introduced to the American and European market - Japanese cartridges are rounded on all four corners of the cartridge, whereas European and American cartridges are rounded on only two. This means that American and European games will fit in a Japanese console, but Japanese games will not fit in an American or European console.
The previous method gets around this by elevating the cartridge connector from inside the console to on top of the inserted cartridge. However, if you cannot afford an extender cartridge, or if you simply don't want the hassle associated with plugging in an extender every time you want to play, it is possible to slice open your cartridge slot doors to accept Japanese cartridges.
This is accomplished by using a warm, sharp knife, and slicing the tabs on the cartridge slot as shown:
You can glue these small cut-off squares to the fold-down doors, so they are a snug fit as before when closed.
The disadvantage to this method is that when you insert a European or American cartridge, you have a small gap around the cartridge slot, that would previously accomodate the Japanese cartridge. So, you open up a new avenue for dust to get into your system. And, you're slicing open your poor system! As with the cartridge slot extender carts, you are only making it easier to play foreign games that are not region-locked - you've solved the hardware problem, but the region lock is in software.
If you have a 32X, the slot on top is large enough for both Japanese (albeit a slightly tight fit) and American/European carts alike.
- METHOD 3 - Game Genie / Pro Action Replay / Pro Action Replay 2
(Skill - minimal, Effort - minimal, Expense - medium, Reliability - somewhat, variable)
These work like Method 2 above, only with the ability to temporarily modify ROM data (Game Genie / Pro Action Replay), or RAM data (Pro Action Replay 2). Remember the 'two values' I mentioned earlier? The game programming that checks these two values is different for every region-locked game. So, by entering certain codes at the Code Entry screen before the proper game starts, you can temporarily overwrite this check so that it returns as OK no matter what console it is plugged into.
On the plus side, this means that so long as you can get the right code, your import will work. Generally, a quick search online for the game name + Game Genie import code will bring up the appropriate information. Sega-16 has a list of the most common import codes here.
On the downside, and from a practical standpoint, you're forced to plug in an additional cartridge whenever you want to play. This means more potential cleaning (with alcohol spray and cotton buds, of course!...), the loss of impulsive play as you need to enter the right code, and of course having to find a GG/PAR/PAR2 unit for sale at a reasonable price. If you have a 32X attached to your Mega Drive, for all our sakes keep it out of high winds.
Also, if a game is designed with a foreign video system in mind, it may play incorrectly or slowly on a machine from a different region. For example - take note, PAL users! - a game designed to run flawlessly at 60Hz NTSC may appear to run slowly on 50Hz PAL Mega Drives. This is because around the time the region lock and TMSS came into existence, Sega presumably instructed its licensed developers how to rely on something other than the video signal for system timing. If you put Sonic 1 on a Japanese machine and Sonic 1 on a European machine side-by-side, the Japanese version runs faster due to the 60Hz NTSC signal, as opposed to the European 50Hz PAL signal. However, if you replace Sonic 1 with a copy of Sonic 3 from the appropriate region, they run at the same speed.
- METHOD 4 - Switchmod
(Skill - medium, Effort - medium, Expense - minimal, Reliability - guaranteed)
This is the final, and most permanent, modification you can do to your console. In a nutshell, instead of affecting how the game reads your console's region, this modification assigns the appropriate values that determine your console's region to two switches.
These values are the video output of your system (i.e. either a 50Hz PAL or a 60Hz NTSC signal), and the internal language setting (i.e. Japanese or Overseas). A Japanese console uses the 60Hz NTSC signal, and the Japanese language setting; a European console uses the 50Hz PAL signal and the Overseas language setting; and an American console uses the 60Hz NTSC signal and the Overseas language setting. No console uses the 50Hz/Japanese setting, as that would be demented.
This method is the only one that requires some level of skill on the owner's part - however, if you can pick up a soldering iron without maiming yourself, you can switchmod a Mega Drive. The hardest part of performing the modification is routing wires and fitting the switches. When the switches are fitted, however, this opens up the entire console's library - owners from PAL territories in particular are likely to enjoy being able to experience region-free games in 60Hz video. Unlike the previously-mentioned methods, you can also experience the changes that some games show when run in different regions - one example is Streets of Rage being renamed Bare Knuckle on a Japanese system.
On the other hand, this modification pretty much irreversably alters your system. If you're the hardcore collector type who collects sealed games and dusts their collection twice weekly, you're probably not going to want to alter your merchandise.
Guides for performing this modification are all over the Internet. Personally, I used mmmonkey's guides to modify my own Mega Drive 2, so I suppose that counts as an endorsement.
And, uh, that's the long and short of it. I've tried to avoid going into the 'whys', and focussed more on the 'how', so those of a more casual approach don't get their heads in a whirl unnecessarily.
 - '50Hz/Japanese' line fixed, thanks for the PMs guys.
[edit2] - Added method 2a, the 'circumcision' method.
[edit3] - TMSS *shakes fist*