A Raspberry Pi (mine is an RPI 3) requires a MicroSD card to boot, but once booted it is not required any longer. This is great because while very convenient, a MicroSD card is not as robust and hardy as a regular SSD card or even a regular USB flash device.
One of the things that I therefore do is to run my RPI’s on either SSD or a USB thumb drive. I’ve also run mine with a 1TB external spinning rust disk with external power. The technique illustrated here works on all of these.
My earlier post described the RPI boot process. The picture here shows a simple MicroSD card image for an RPI. The disk is partitioned into two parts, the first partition is a small FAT32 LBA addressable partition and the second is a larger ext4 partition. The FAT32 partition contains the bootloader and the ext4 partition contains the root filesystem.
The thing that these two together is cmdline.txt which defines the root device with a declaration like:
Since the RPI always mounts the MicroSD card as /dev/mmcblk0 and the partitions are numbered p1, p2, and so on, this indicates that the root partition is the ext4 partition as shown above.
To move this to a different location is a mere matter of adjusting cmdline.txt (and updating /etc/fstab) as shown below.
Here is my RPI with a USB thumb drive running the root (/) filesystem.
As you can see, I have a USB drive which shows up as /dev/sda and the MicroSD card.
amrith@rpi:~$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT sda 8:0 1 115.7G 0 disk └─sda1 8:1 1 115.7G 0 part / mmcblk0 179:0 0 29G 0 disk └─mmcblk0p1 179:1 0 100M 0 part /boot amrith@rpi:~$ blkid /dev/mmcblk0p1: LABEL="BOOT" UUID="E4F6-9E9D" TYPE="vfat" PARTUUID="01deb70e-01" /dev/sda1: LABEL="root" UUID="7f4e0807-d745-4d6e-af6f-799d23a6450e" TYPE="ext4" PARTUUID="88578723-01"
I have changed cmdline.txt as shown below.
amrith@rpi:~$ more /boot/cmdline.txt [...] root=/dev/sda1 rootfstype=ext4 [...]
and updated /etc/fstab (on the USB drive) as shown below.
amrith@rpi:~$ more /etc/fstab # fstab generated by rpi-base element proc /proc proc defaults 0 0 LABEL=BOOT /boot vfat defaults,ro 0 2 PARTUUID=88578723-01 / ext4 defaults,noatime 0 1
As you can see, I’ve also marked the MicroSD card (which provides /boot) to be readonly; in the unlikely event that I have to modify it, I can remount it without the ‘ro’ option and make any changes.
On the left hand side is the MicroSD card. Note that on the MicroSD card, in the FAT32 partition, cmdline.txt is in ‘/’ (there’s no /boot on the MicroSD card). the cmdline.txt points the root partition to /dev/sda1 which is the USB flash drive.
On the right hand side is the USB flash drive, it has an ext4 partition with an /etc/fstab entry which mounts the MicroSD card’s fat32 partition on /boot and mounts itself on /.
This works just as well with any external disk; just make sure that you have adequate power!