Forked from https://github.com/gfoot/sdcard6502/ and modified to work with my PAL-1. Code can be assembled with ca65.
This guide shows how to connect up an SD card reader to a PAL-1 6502 and use it to read/write data from SDHC cards.
The tutorial ends with being able to read arbitrary sectors from the SDHC card. Beyond that, I've included library-style code that can read/write from a FAT32 filesystem, with some example programs. For more information on this library code, see FAT32 Library Documentation
I'm only covering SDHC cards. You pretty much can't buy older/smaller cards these days, and not bothering to support the older protocol simplifies the code quite a bit.