How to use the external flash of Pro Micro RP2040

Hello, how can I use the 16 MB external flash of the Sparkfun Pro Micro RP2040?

If I mount the board on my Mac (Big Sur), it shows some strange behaviour. The Mac says, that 669 MB are free, what obviously is wrong. I can copy files up to 2 MB onto it. I guess, thats the internal memory of the RP2040. Trying to copy bigger files fails with an unknown error (error code 100093)

This might help.

https://kevinboone.me/picoflash.html?i=1