I am working with the FATFS r14 using long file names. and most all is working OK. I am more than occasionally seeing FR_NOT_READY responses on f_opendir, etc. Any ideas??

[Tesla DeLorean]

Instrument DISKIO to understand the source/sequence of the error.

Look at the ErrorCode signalled by the SDIO/SDMMC layer.

Make sure writes complete properly.

Make sure media-present detect pins are not flagging the card is missing.