The GD-ROM upgrade is compatible with Naomi and Naomi 2 via the option port on the top of the board. It comes with a DIMM board, very similar to the ROM board on a normal NAOMI cartridge, but has RAM instead of ROMS.
RAM board Uses normal PC type DIMM's, so upgrading memory is not a problem, and relativly cheap.
The GD Rom system has a very clever way of getting around the loading delay time. When the game is initially installed, the GD ROM loads its content into the Naomi RAM. Once this is completed, the GD ROM goes into standby unless the RAM gets corupted or the game gets shut off for 72 hours or more. This does two things,
1) Eliminates loading delays completely, (after initial power up.)
2) Minimizes the on/off cycles of the GD ROM drive and greatly increasing the life of this mechanical device.
From Sega R+D - "We learned a lesson watching our coin-op competitors go through hardware hell. Many have been using off-board storage devices for years - hard drives, CD's, etc. with great success in increased content but at a tremendous cost in realiablity. These storage devices simply could not withstand the constant on/off cycling a coin-op application demands. Add to that the dirt and smoke a game has to put up with on location vs. a normal computer application and you can see how this was a problem waiting to happen"
The GD-Rom system uses an electronic "key" that plugs into the DIMM module to act as an activator, to prevent people just copying the GD-Roms, visible on the top right of the pictures. A different key is supplied with every game.
There is a well founded rumor that Sega may use this system for having multigame cabinets like the neogeo multi systems, as the GR-Rom can store a LOT of data....