Sega Master System / Mark III / Game Gear
The Master System and Mark III were equipped with a card slot into which may be inserted credit-card sized software "cards", which contain thin ROM chips of up to 32KB in size . These were marketed under the "Sega My Card" (Japan) and "Sega Card" (rest of world) brands.
The SG-1000 and SC-3000 could also use cards through the "Card Catcher" cartridge slot adaptor.
If a card is inserted at the same time as a cartridge on a Master System, the card will take priority. On the Mark III, the cartridge takes priority.
Viewing the card media from above, such that the label would face the viewer:
Applies to: All Sega Card and MyCard games for Master System and SG-1000, released worldwide.
The card slot can be selected using port $3E on the Master System. However, the BIOS is generally responsible for selecting the correct slot and thus software generally does not need to bother with this.
No known software, apart from BIOSes, access the card slot when run from other physical locations.
The export Master System uses the card slot for the 3D Glasses Adaptor. This is not acting as a card, merely using the slot to gain read-only access to the memory bus. Address FFF8 with mirrors up to FFFB bit 0 toggles the shutter glasses. Writes end up in ram and hardware, reads come from ram only.