I also recall that 5.2.1 was working. 5.3.0 was when it failed the SdFat. But just going back to 5.2.1 or 5.2.0 didn't fix. Had to go way back to 5.0.0 and also delete the temp files.

I've just done the RP2040SDIO test (parallel SDIO, way faster than 1-bit SPI) and it ran fine. As did the bench example using both SDIO and plain SPI1.

My bench.ino diffs are below. All I did was set the proper CS pin, force FatType to 3 (so it uses SdFs like in your snippet), and set up the SPI1 port (I have a hardwired testboard w/this config).

earle@amd:~/Arduino/hardware/pico/rp2040$ diff ~/Arduino/bench/bench.ino libraries/SdFat/examples/bench/bench.ino 
10c10
< #define SD_FAT_TYPE 3
---
> 
32c32
< const uint8_t SD_CS_PIN = 13;
---
> const uint8_t SD_CS_PIN = SS;
58,60d57
< #define SD_CONFIG SdSpiConfig(SD_CS_PIN, DEDICATED_SPI, SPI_CLOCK, &SPI1)
< 
< 
144,147d140
< SPI1.setCS(13);
< SPI1.setMISO(12);
< SPI1.setMOSI(11);
< SPI1.setSCK(10);

That said, you really should consider using SDFS or SD from the core (which use the SdFat library underneath) to be compatible with the internal core File, Dir, etc.

5.0.0 brought in an updated SdFat library from upstream, and there was a minor change in 5.2.1 allowing SPI to be unidirectional, but otherwise that part of the core has been stable.

Thanks for looking into this. :-) I did a clean install of V5.0.0 and it worked. Updated to V5.3.0 and it stops working. Did another clean of V5.0.0 and it's back again. I will do more tests soon. Cheers.

BTW: right now I can't use SDIO, as I ran out of pins on my project. :-o