how to use USB hard disk as /(root)

[hatahata]

hi all .

i usually use hard disk as root .
i think   hard disk is more tough than sd card  and the space of it is over 250GB or more .

for example banana pi M1 , root is /dev/sda1

Code:

head /boot/boot.cmd
if ext4load mmc 0 0x00000000 /boot/.verbose
then
setenv verbosity 7
else
setenv verbosity 1
fi
setenv bootargs "console=tty1 root=/dev/sda1 rootwait rootfstype=ext4 cgroup_enable=memory swapaccount=1 hdmi.audio=EDID:0 disp.screen0_output_mode=1920x1080p60 panic=10 consoleblank=0 enforcing=0 loglevel=${verbosity}"

but
at pine64's debian  , surely there are files in /boot  but they are binaly files  so i do not do anythig , i give up .
only develover knows how to  , so i think .
regards