The firmware doesn't change anything related to the SD Card, and nobody else reported an issue like this, so it has to be something else entirely.
If you want to get to the stock firmware, you need to install the corresponding StockRevert package, depending on the currently installed firmware (check the Unit Information menu).
You can find a link to previous StockRevert packages
here, or you can find the StockRevert for the Walkman One firmware in the internal storage of the player, in the CFW folder. After running the StockRevert package, install the 1.02 update, which can be found in the same CFW folder in the internal storage of the player.