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I know it's not possible to directly boot from an external USB stick / drive, but instead you have to boot from the SD card and then the external device can take over. What's the easiest and preferred way to set this up, therefore boot from SD card and then let an external device take over? Thanks for any hint!

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4 Answers

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If you have an existing OS running on the Pi, then firstly it would be useful to know if the USB device is supported. You can do this by mounting it like normal:

mount /dev/sda1 /mnt

If that fails then you wont be able to use the USB device as a root partition without enabling the kernel modules for it. And for that you may need to compile your own kernel.

If it suceeds then it should work fine with some tweaking of the boot parameters that the Pi uses:

On an existing image, open cmdline.txt, which can be found on the boot partition, and enter the following lines:

dwc_otg.lpm_enable=0 console=ttyAMA0,115200 kgdboc=ttyAMA0,115200 console=tty1 
root=/dev/sda1 rootfstype=ext4 rootwait text

All you must then do is flash that image to the SD card and boot the Pi. If all is well, /dev/sda1 should be the location of the USB drive when the Pi boots, and thus it should attempt to use that location as root. The rootwait parameter is important as it will make the boot process hang until the USB drive is recognised. Without it the Pi may complain that the location doesn't exist.

I suggest that you copy the root partition from an existing Raspberry Pi image to your USB drive and use that to boot from.

Let me know if you need any further information.

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+1 Great answer. Shouldn't console=... already be there? – Alex Chamberlain Jul 21 '12 at 14:02
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Also, can you use UUIDs on the command line? – Alex Chamberlain Jul 21 '12 at 15:27
Yes I provided the whole file and where are you talking about using UUIDs? – Jivings♦ Jul 21 '12 at 18:01
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Thank you very much Jivings, I "flashed" the official image to my USB stick, then copied the stuff from the FAT partiiton to an SD card and adjusted the cmdline.txt. In my case the root partition is then /dev/sda2, because /dev/sda1 is the (unused) boot partition on the usb stick. I didn't need the rootwait in my case, but good to know this parameter! – stefan.at.wpf Jul 21 '12 at 18:14
@stefan.at.wpf (and Jivings) You can use UUIDs instead of /dev/sdx - at least in /etc/fstab/ - then it doesn't matter if the x changes. – Alex Chamberlain Jul 22 '12 at 16:28
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The easiest way to set this up would be to use BerryBoot. It supports installation to, or from, one or more images on a USB stick (or on the SD card itself) out of the box through a nice GUI interface.

To set it up all you have to do is copy the ~70MB of install files on to a FAT32 formatted SD card, insert it into your device and boot away. It even has a list of preconfigured images that it can automatically download and install for you over an ethernet or wi-fi connection.

You then select the OS you would like to be booted by default from your available images, and if you want to add new images or change the default at a later date you simply hit enter during the boot process to reconfigure!

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move the image to the SD card ? I thought we want to install it on a USB stick ? – Suhaib May 12 at 21:57
As the OP stated it's not possible to boot directly off a USB stick on a Raspberry Pi - You need an SD card with a bootloader installed that can hand the process over to another device once powered up. – Nathan Dunn May 14 at 0:19
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I prefer to use e2fs labels for partitions to UUIDs when not using lvm.

e2label /dev/sda2 USBROOT for instance and then in /etc/fstab use :

LABEL=USBROOT / ext4 ...

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Hi Nox, Welcome to Stack Exchange! May I suggest you include something like "Further to @Jivings answer,...", so that beginners understand you are providing an alternative way to refer to disks? – Alex Chamberlain Oct 24 '12 at 17:03
up vote 1 down vote

ok, to clarify -

further to @Jivings answer -

There are several ways that you can refer to a disk in the fstab. Using /dev/sdXX might not always be safe, as the order that scsi disks appear may not be guaranteed.

What I do not know, is if the boot configuration understands anything but basic device labels. "root=/dev/sda1" is an abstraction however, so in theory root=LABEL=TEST might work. I need to investigate.

I apologize if I am not being very clear. I've got some skill gain to do in explaining linux to new folks, obviously.

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