Pi looks in to the first partition in the boot process, it reads the cmdline to know where to find the rootfs. We need to change in the SSD /etc/fstab so that we mount the SD boot partition for the firmware updates.

• change cmdline.txt to point to the ssd partition

console=serial0,115200 console=tty1 root=PARTUUID=4811dfd1-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait

• in the ssd /etc/fstab point to the PUUID of the SD boot

root@pi4fr:/boot# cat /etc/fstab
proc            /proc           proc    defaults          0       0
PARTUUID=738a4d67-01  /boot           vfat    defaults          0       2

• blkid info

root@pi4fr:/boot# blkid
/dev/mmcblk0p1: LABEL_FATBOOT="boot" LABEL="boot" UUID="4BBD-D3E7" TYPE="vfat" PARTUUID="738a4d67-01"
/dev/mmcblk0p2: LABEL="rootfs" UUID="45e99191-771b-4e12-a526-0779148892cb" TYPE="ext4" PARTUUID="738a4d67-02"
/dev/sda1: LABEL_FATBOOT="boot" LABEL="boot" UUID="3FFE-CDCA" TYPE="vfat" PARTUUID="4811dfd1-01"
/dev/sda2: LABEL="rootfs" UUID="3122c401-b3c6-4d27-8e0d-6708a7613aed" TYPE="ext4" PARTUUID="4811dfd1-02"
/dev/mmcblk0: PTUUID="738a4d67" PTTYPE="dos"
root@pi4fr:/boot#