fat16lib:
Many SD cards require chip select to be held low for the entire 512 byte block write. You can't suddenly switch use of the hardware SPI bus during a block write. SD cards only allow CS for go high at certain times.
This is why I use software SPI for the ADC in the interrupt code.
You will need to use software SPI for the SD, which may be too slow or use software SPI for the CAN bus.
So my guesswork went into the right direction ... sadly ...
Thanks a million for your quick answer!
I'll reconsider my options and use your logging code for now (it's already extremely performant, even when disabling interrupts while writing!).
Keep up the great work fat16, your SD library already works tremendously well! Thank you!