Hi Dan,
Just FYI, SPI devices use one of four modes for data transfer. The mode sets the clock polarity and edges. u-blox GNSS use MODE0, MSBFIRST. You’re OK because the v3 GNSS library automatically (re)sets the mode for each transfer. Not all libraries do that. It’s just something to watch out for when using multiple SPI devices - which may use different modes.
You need to tell SdFat that it doesn’t have exclusive access to the SPI bus. Please try:
if (!SD.begin(SdSpiConfig(CS_SD, SHARED_SPI, SD_SCK_MHZ(8))))
If you want to use larger SD cards (FAT16, FAT32, exFAT), tell SdFat to use:
SdFs SD;
FsFile myFile;
I hope this helps,
Paul