Well think @KurtE is right, it may be more complicated. Yesterday, using a sketch I did form giga mbed ( Configuring SPI Speed For Use In SD Library On Arduino GHiga - Hardware / GIGA R1 - Arduino Forum ) where I define my own driver using transactions - note didn't see anything in the zephyr core for a typical define SPI_HAS_TRANSACTIONS.
Got it working yesterday and was able to read the SD Card but the CS pin was hard coded to 10. So went back in to try to fix that and then it hung again. So I reversed the changes and then the Giga zephyr is hanging again using SDFat. Something is still off - could be timing or something else.
Right now though all is silent Arduino Team so really just shooting in the dark.