Is this line correct for NX devkit with 64gb samsung sdcard for full flashing from folder created by sdkmanager?

sudo  -S 64GiB x avier-nx-devkit mmcblk0p1

!NB flashing without the -S argument results in 14GB sdcard formatting

Try with a smaller size. The “-S size” is for the rootfs partition…if you fill the card with that, then there is no room for other content/paritions. E.g., try with “-S 56GiB”.