My Japanese PS2 has a PC-Card Slot
As it turns out, Japanese Playstation 2 models have a PC-Card (PCMCIA) slot on the back instead of that large expansion bay for IDE hard drives that we have here in the states.
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As far as I can tell online, the PCMCIA slot is also used for storage expansion, except instead of using a gigantic IDE drive you can use a compact flash card.