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Boot Raspberry Pi from USB

This how-to will show you how to begin using an external USB device - say a USB thumb drive or an External HD - as, not the "boot" device, but rather the storage location for the system files.

Find the USB Drive

If you're using a simple low-powered thumb drive, simply plug it into the Raspberry Pi. If you're planning on using a larger, HDD-like drive, I would suggest you use an externally powered USB hub device - to help power the external drive and not draw too much from the Raspberry Pi.

Next, type lsusb to see a basic break down of what your RPI can see. It doesn't exactly get you a lot of info, but it doesn't hurt to start here.

Usually, I see:

Bus 001 Device 004: ID 0930:6545 Toshiba Corp. Kingston DataTraveler 102 Flash Drive / HEMA Flash Drive 2 GB / PNY Attache 4GB Stick
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. SMSC9512/9514 Fast Ethernet Adapter
Bus 001 Device 002: ID 0424:9514 Standard Microsystems Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

The top line being the flash drive that I have plugged in. But, you'll notice, it doesn't give you the nice pretty /dev/sd[x] that we're needing here.

Now, the website I'm using as a guide for this instructional [AFRUIT-USB] suggests using dmesg as a way to find your device. Though this can be messy because dmesg is basically your systems "this is what I'm doing and seeing all the time" kind of messaging service. Anytime a USB device is plugged or unplugged, anytime you start, stop or restart your machine, or any number of events occurring, it adds itself to dmesg. So, don't get flustered if you can't find your USB device.

If the last thing you did was insert your USB, it should be the last item appearing on dmesg

Using sudo dmesg should show something resembling:

[  459.896922] usb 1-1.2: new high-speed USB device number 4 using dwc_otg
[  460.018734] usb 1-1.2: New USB device found, idVendor=0930, idProduct=6545
[  460.026951] usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[  460.035530] usb 1-1.2: Product: TransMemory-Mx
[  460.041271] usb 1-1.2: Manufacturer: TOSHIBA
[  460.046801] usb 1-1.2: SerialNumber: 60A44C429E6BED81F000ED00
[  460.055140] usb-storage 1-1.2:1.0: USB Mass Storage device detected
[  460.064034] scsi host0: usb-storage 1-1.2:1.0
[  461.068295] scsi 0:0:0:0: Direct-Access     TOSHIBA  TransMemory-Mx   PMAP PQ: 0 ANSI: 6
[  461.081327] sd 0:0:0:0: [sda] 60929280 512-byte logical blocks: (31.2 GB/29.1 GiB)
[  461.093654] sd 0:0:0:0: [sda] Write Protect is off
[  461.100343] sd 0:0:0:0: [sda] Mode Sense: 45 00 00 00
[  461.101667] sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesnt support DPO or FUA
[  461.114459] sd 0:0:0:0: Attached scsi generic sg0 type 0
[  463.171778]  sda: sda1
[  463.182905] sd 0:0:0:0: [sda] Attached SCSI removable disk

You'll notice the sda monikers there. That would be the /dev/sda that we're looking for. So, the newest USB device has been assigned /dev/sda on my machine. Now, of course, yours might be different. You never know.

Or, if you're dmesg is stupidly long and you can't find it quickly, you can always sudo dmesg | grep sd[a-z]

grep being a searching program and the sd[a-z] being whats called a "regex" search. Basically, grep takes sd then adds each letter from a-z to the end and searches for those terms. It then will present you with the information, making it easier for you to find things. The same thing works with [0-9] as well. I like to use it to delete a large amount of log files that tend to build up inside of /var/log. sudo rm ./*.[0-9].* or what have you.

Install Adafruit's USB Program

This specific program they've written automates the wiping and partitioning of the USB drive, moving of the system files, and setting up the boot files.

Adafruit's apt-get Repo Script

So, to install it, you'll want to add Adafruit's Linux Repo to your apt-get stuffs. You can either:

curl -sLS https://apt.adafruit.com/add | sudo bash

Which will automatically add their repo and do a sudo apt-get update for you. Or, you can go the long way round:

Adafruit's Repo the Long Way Round

  • First, adding the repo to your sources.list.d
echo "deb http://apt.adafruit.com/raspbian/ jessie main" | sudo tee  /etc/apt/sources.list.d/adafruit.list
  • Then, grabbing their gpg key:
wget -O - -q https://apt.adafruit.com/apt.adafruit.com.gpg.key | sudo apt-key add -
  • And then run sudo apt-get update

Now, before we go to far, why don't we break all that down?

  1. The echo deb http://apt.adafruit.com/raspbian/ jessie main part first
  • echo so that we can copy and pipe that quoted text
  • sudo tee is a way to take text thats been piped into it, and either overwrite or append that text to a file. In this case, overwrite to /etc/apt/sources.list.d/adafruit.list
  1. The GPG Key
  • wget is a program to download things from the interwebs
  • -O - -q: - the -O is for amending the output of the download - the - is saying the output is STDOUT, or copy it to output so we can pipe it - the -q is a flag saying "run it quietly" or "no output preferred other than the file"
  • sudo apt-key add - is to add the downloaded key, and the - is saying "take the STDOUT from the wget and use that"

Installing USB Program

Next, you'll want to install Adafruit's USB program.

sudo apt-get install adafruit-pi-externalroot-helper

Running the Program

Then, once you know for sure the file location moniker of your USB device:

sudo adafruit-pi-externalroot-helper -d /dev/sda

The -d flag is to tell it the file location moniker of your USB device.

Note

Make SURE you get this right, as you don't want to really wipe your SD card. Though I'm fairly certain not only would that not work, it luckily is an easy fix to get either a clean OS on it, or a backup you've made.

This program does take at least a few minutes to run through. You'll need to restart your RPI as well once its finished.

After Running

It will give you a series of messages once its done. Make sure to read through them, in case there are any errors. It also tells you how to make sure it worked:

[boot config] Ok, your system should be ready. You may wish to check:
[boot config]   /mnt/etc/fstab
[boot config]   /boot/cmdline.txt
[boot config] Your new root drive is currently accessible under /mnt.
[boot config] In order to restart with this drive at /, please type:
[boot config] sudo reboot

fstab

The key lines are:

Ok, your system should be ready. You may wish to check:
    /mnt/etc/fstab
    /boot/cmdline.txt

It really means you need to check /etc/fstab. Not sure why they included the extra /mnt in there.

My /etc/fstab now shows:

/dev/mmcblk0p1 /boot vfat defaults 0 2
#/dev/mmcblk0p2 / ext4 errors=remount-ro,noatime,nodiratime,commit=120 0 1
tmpfs /tmp tmpfs defaults,nodev,nosuid 0 0
/dev/disk/by-uuid/94551cfd-d0fc-42df-b742-b7a6434c0d8a    /   ext4    defaults,noatime  0       1

Notice the commented out line #/dev/mmcblk0p2

SD Card Info

That was the original line for the sd card. The line prior was also there before, as the SD card was both the boot media and the OS media. But, now the SD is ONLY the boot, and the external USB is the OS media.

Running df -h

You can also df -h. This shows your filesystem stuffs. The -h being human readable format on the sizes.

Filesystem      Size  Used Avail Use% Mounted on
/dev/root        29G  1.2G   26G   5% /
devtmpfs        483M  4.0K  483M   1% /dev
none            4.0K     0  4.0K   0% /sys/fs/cgroup
tmpfs           487M     0  487M   0% /tmp
none             98M  232K   98M   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            487M     0  487M   0% /run/shm
cgmfs           100K     0  100K   0% /run/cgmanager/fs
none            100M     0  100M   0% /run/user
/dev/mmcblk0p1   61M   36M   26M  58% /boot
tmpfs            98M     0   98M   0% /run/user/1000

The location /dev/root is the USB drive now. And, it shows the large size of the USB drive as well.

Recovering from a Failed Boot

If the RPI should ever not wanna work correctly with the USB drive this way, you can always:

  1. take the SD card out of your RPI, plug it into your regular machine that you used to install the OS.
  2. Then, open the first partition and find the file cmdline.txt.
  3. Replace the text root=PARTUUID=....rootdelay=5 with root=/dev/mmcblk0p2, which will point the root partition back to the 2nd partition of your SD card. It should roughly look like this:
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait fbcon=map:10 fbcon=font:VGA8x8

Then, place the SD card back into the RPI, and it should boot normally.

[AFRUIT-USB]Adafruit's Raspberry Pi on USB
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