Using a USB drive as OS root on a Raspberry Pi
2012-12-29
This is how you change your root file system to use a USB drive on a Raspberry Pi.
First, let's make sure you know which partition is your root file system right now. Enter this command in a terminal on your Raspberry Pi:
df -h
It should say something like this:
treddell@penelope ~ $ df -h
Filesystem Size Used Avail Use% Mounted on
rootfs 15G 1.9G 13G 14% /
/dev/mmcblk0p2 15G 1.9G 13G 14% /
devtmpfs 243M 0 243M 0% /dev
tmpfs 49M 224K 49M 1% /run
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 98M 0 98M 0% /run/shm
/dev/mmcblk0p1 56M 17M 40M 30% /boot
Notice that /dev/mmcblk0p2 is th same size as rootfs? That's your root partition.
Second, you need to copy your current root partition from the SD card to the USB drive. Execute this command with only your single USB drive inserted, no other drives (other than the SD card):
sudo dd if=/dev/mmcblk0p2 of=/dev/sda1 bs=4M
Depending on how large your SD card is, this could take a long time.
Next, we want to check the USB drive for errors with this command:
sudo e2fsck /dev/sda1
If that turns out okay, we can resize your new root partition to fill your USB drive:
sudo resize2fs /dev/sda1
Now you should mount your USB drive so that we can edit your future fstab file:
sudo mount /dev/sda1 /mnt
To edit the fstab, use nano with this command:
sudo nano /mnt/etc/fstab
It will look something like this:
proc /proc proc defaults 0 0
/dev/mmcblk0p1 /boot vfat defaults 0 2
/dev/mmcblk0p2 / ext4 defaults,noatime 0 1
# a swapfile is not a swap partition, so no using swapon|off...
You need to change it to look like this:
proc /proc proc defaults 0 0
/dev/mmcblk0p1 /boot vfat defaults 0 2
/dev/sda1 / ext4 defaults,noatime 0 1
# a swapfile is not a swap partition, so no using swapon|off...
So that /dev/sda1 is mounted as root (/).
Next, unmount the USB drive:
sudo umount /dev/sda1
And edit the cmdline.txt file in your boot partition with this command:
sudo nano /boot/cmdline.txt
You will want it to look like this:
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 root=/dev/sda1 rootfstype=ext4 elevator=deadline rootwait
Notice that root=/dev/sda1.
Now you can reboot (sudo reboot) and you should be good to go with your new root partition on the USB drive. You can check it with the df -h command we used earlier and see the new size of rootfs.