A bit of self promotion
I made a software suite for interfacing with a PlayStation Memory Card using Arduino boards.
You can find it here
. It's called MemCARDuino and it's open source.
PS1 Memory Cards use a slightly modified SPI protocol for communication with a set of
predetermined commands (read block, write block, etc).
Also, Memory Cards only have 128 KB (1 Megabit) of storage space so it's nothing fancy.
While there is a file system nothing stops you from writing 128 KB of your own data, it just won't be readable on a PS1.