Hi neil57,
The perquisites to using SD to update bootloader is that the bootloader in QSPI is older than the one from SD image.
Could you check if the following workflow works?
Step 1. Insert SD Card and run the following command to flash both QSPI and SD with R35.3.1
$ sudo ./tools/kernel_flash/l4t_initrd_flash.sh --external-device mmcblk0p1 -c tools/kernel_flash/flash_l4t_t234_nvme.xml \
-p "-c bootloader/t186ref/cfg/flash_t234_qspi.xml" --showlogs --network usb0 jetson-orin-nano-devkit internal
Step 2. Prepare another SD card with SD image of R35.5.0
Step 3. Replace the SD card with the one created in Step2
Step 4. Check if the bootloader in QSPI get update after you run reboot command