Boot RaspberryPi 3B/3B+/4 from SD card but have everything else on SSD

Question

I have some Raspberries (3B/3B+/4) that I want to boot from SSD, but I also want to attach an HDD to them (the classic SSD for OS and HDD for data)

The thing is that this setup has give me a lot of headaches in the past and nowadays too. When the raspberry boots up, if it finds the SSD first, then everything works correctly. But, if for some reason it finds the HDD first, it won't boot up at all.

As long as the SSD is detected first no problem, but on power outages or reboots it can happen that it won't boot because it found the HDD first which is not bootable.

I couldn't fix that after a lot of tinkering so I was wondering if it could be a viable option to set only the boot partition on the SD card then have the rest of the OS on the SSD and the data in the HDD.

I think some people have managed to do this, but I couldn't find much literature/tutorials on how to do it.

How can I do this?

Thanks in advance

Answer 1

Here's one way

create a file, lets call it sd.txt, with the following content

label: dos
device: /dev/mmcblk0

/dev/mmcblk0p1 : start=        8192, size=      524288, type=c

Now, insert your SD card, and run

sudo sfdisk /dev/mmcblk0 < sd.txt

Now run

blkid

Look for the line that starts with /dev/mmcblk0p1 - should look something like this (with different values for the UUID/PARTUUID of course)

/dev/mmcblk0p1: LABEL_FATBOOT="boot" LABEL="boot" UUID="0123-4567" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="b000b135-01"

and take note of the PARTUUID

Edit (as root) /etc/fstab and find the line that looks like

PARTUUID=deadbeef-01  /boot           vfat    defaults,flush    0       2

and replace the PARTUUID (deadbeef-01 in the example) with the PARTUUID you found before (b000b135-01 in this example) - note, do NOT include any quotes (") i.e. in this example it'll change to

PARTUUID=b000b135-01  /boot           vfat    defaults,flush    0       2

Now copy the boot partition from the SSD to the SD card

sudo dd if=/dev/sda1 of=/dev/mmcblk0p1

Reboot and enjoy