Booting a Raspberry Pi From a 16 MB SD Card

By Craig Thomas, Sun 18 January 2015, in category Development operations

raspberry_pi

Raspberry Pis make great little servers. With low power consumption, they make great devices if you need cheap web services running 24/7. One aspect I dislike about them however, is that they need to boot using an SD card. Given the limited number of write cycles that most SD cards have, I hate having to run anything more than the bootloader off of one.

While there are many tutorials that talk about how you can clone the SD card to a USB drive, I wanted to do something a little different. Why waste a 4 GB SD card? I have lots of smaller SD cards kicking around. In this post, I’ll discuss how I made my Raspberry Pi boot from a 16 MB SD card and use a USB hard drive for the root filesystem.

Running the Pi from a USB Hard Drive

Most tutorials have you simply clone your SD card to a USB hard drive, and then edit the cmdline.txt file to point to the USB hard drive when booting the main OS. This solves the problem of moving the root filesystem to a more robust storage medium, but ties up a 4 GB SD card that I could be using elsewhere. I know this isn’t too big of a problem, but it does mean buying a bunch of 4 GB SD cards for all my Raspberry Pis.

On a related note, I have a number of digital cameras that I have purchased over the years, and most of them came with a 16 MB SD card. Back in the day, 16 MB was a lot of storage space, but given the megapixel counts that come with most cameras, 16 MB can usually only store a few pictures. In my case, they sit in a stack in my desk drawer, gathering dust and sulking.

This got me thinking: I’d really love to use the 16 MB card in the Raspberry Pi (plus a hard drive), and use the 4 GB card in my cameras.

*Update January 20, 2015: Mike Redrobe discussed on his blog how to do this in May 2014, with great instructions on how to do this under Windows. Also, this blog noted how to do it in 2012, but without the step-by-step breakdown. Many other sources (like this blog, and this blog) outline how to modify the boot partition to mount the root filesystem from a USB drive.

What is Where

I took one of my 4 GB SD cards that had Raspbian on it, and examined how the card was formatted. There is a small MS-DOS partition used for the bootloader, and the rest of the card is used for the root filesystem. The MS-DOS partition is what interests me, because this is what the Raspberry Pi needs to see in order to bootstrap the OS. Here’s the contents:

# ls -lh /boot
total 9.7M
-rwxr-xr-x 1 root root  18K Dec 21 10:53 bootcode.bin
-rwxr-xr-x 1 root root  120 Dec 21 11:14 cmdline.txt
-rwxr-xr-x 1 root root 1.3K Dec 21 11:14 config.txt
-rwxr-xr-x 1 root root 2.3K Dec 21 10:53 fixup_cd.dat
-rwxr-xr-x 1 root root 6.0K Dec 21 10:53 fixup.dat
-rwxr-xr-x 1 root root 9.0K Dec 21 10:53 fixup_x.dat
-rwxr-xr-x 1 root root  137 Dec 24 12:29 issue.txt
-rwxr-xr-x 1 root root 3.2M Dec 21 10:53 kernel.img
-rwxr-xr-x 1 root root  19K Sep 25  2013 LICENSE.oracle
-rwxr-xr-x 1 root root 538K Dec 21 10:53 start_cd.elf
-rwxr-xr-x 1 root root 2.6M Dec 21 10:53 start.elf
-rwxr-xr-x 1 root root 3.5M Dec 21 10:53 start_x.elf

Most of those files look pretty small... hmm... I wonder how much disk space is actually used:

# df -h .
Filesystem      Size  Used Avail Use% Mounted on
/dev/sdc1        56M  9.7M   47M  18% /mnt/raspbianboot

So on the 60 MB MS-DOS boot partition, 9.7 MB worth of files are used for the bootloader. This is perfect! I can fit those files on a 16 MB SD card!

Preparing the 16 MB SD Card

Step 1 was to format the 16 MB SD card as needed in order to boot Raspbian. So, I stuck the card in the computer and took a look at the partitions on it (under Ubuntu, the device registered as /dev/sdc).

Disclaimer: I use fdisk to delete partitions on the card. If you accidentally specify the wrong device, you can destroy your hard disk partition and make your computer unbootable. These instructions are for educational purposes only. Running any of the commands below is at your own risk!

sudo fdisk -l /dev/sdc

Disk /dev/sdc: 16 MB, 16056320 bytes
2 heads, 32 sectors/track, 490 cylinders, total 31360 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1              57       31359       15651+   1  FAT12

Okay, so this is the correct device – 16 MB total storage. It’s currently formatted as FAT12. For the Raspberry Pi to boot, we need a FAT32 filesystem instead. So, I started up fdisk and deleted the primary partition:

# sudo fdisk /dev/sdc

Command (m for help): d
Selected partition 1

Next, I created a new primary partition on the card:

Command (m for help): n
Partition type:
   p   primary (0 primary, 0 extended, 4 free)
   e   extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-31359, default 2048): 
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-31359, default 31359): 
Using default value 31359

Next, I changed the partition to W95 FAT32 (LBA). This is type ID c:

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): c

Finally, I needed to mark the new primary partition as bootable:

Command (m for help): a
Partition number (1-4): 1

With that done, I saved the partition information, and quit fdisk:

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.

Note: if you receive messages about the system retaining the old partition information after you create the new primary, simply remove the card from the card reader, and then re-insert it.

Finally, I created the filesystem on the new partition:

sudo mkfs -t vfat /dev/sdc1

Copying the Boot Files

The next step was to copy the boot files from the original Raspbian 4 GB card to the 16 MB card. I removed the 16 MB card from my card reader, and popped in the Raspbian 4 GB SD card. I created a mount point for it, and mounted the boot partition. Note that the first partition on the card (again registered as /dev/sdc) contains the boot partition. So /dev/sdc1 is the boot partition, while /dev/sdc2 contains the root filesystem:

sudo mkdir /mnt/raspbianboot
sudo mount /dev/sdc1 /mnt/raspbianboot

I needed a temporary location to store the files, so I copied them to /tmp:

sudo cp -r /mnt/raspbianboot /tmp

So far so good. Now to put them onto the 16 MB card. I unmounted the boot partition:

sudo umount /mnt/raspbianboot

I popped out the Raspbian 4 GB SD card and put in the 16 MB card. To keep things clear, I created a new mount point for it... just so I didn’t confuse myself with what I was mounting. Again, when I took out the 4 GB card and put in the 16 MB card, it registered as /dev/sdc:

sudo mkdir /mnt/raspbianboot16
sudo mount /dev/sdc1 /mnt/raspbianboot16

Next, I copied the files from /tmp to the 16 MB SD card:

sudo cp -r /tmp/raspbianboot/* /mnt/raspbianboot16

The final thing was to modify the boot loader on the 16 MB card to boot from the USB hard drive instead of trying to boot on the second partition of the SD card (which would be bad, since there isn’t one!):

sudo vi /mnt/raspbianboot16/cmdline.txt

I changed:

root=/dev/mmcblk0p2

To the following:

root=/dev/sda1

I also added two additional flags to the file at the end of the line:

bootdelay rootdelay

I saved the file and unmounted the drive:

sudo sync
sudo umount /mnt/raspbianboot16

Cloning Raspbian Wheezy to the USB Hard Drive

The last piece of the puzzle is to clone the other Raspbian root filesystem on the SD card to the USB hard drive. I have a 320 GB USB drive that I use for things like my Deer Detector. First up, I put the Raspbian 4 GB SD card back into my card reader, which registered as /dev/sdc. Next, I plugged in the new USB hard drive. In my machine, the new drive shows up as /dev/sdd. I copied the root partition from the Raspbian 4 GB SD card to the new disk. Note that since I want the root partition, it’s /dev/sdc2 that I want to copy from (remember /dev/sdc1 was the boot partition):

sudo dd if=/dev/sdc2 of=/dev/sdd bs=4M

One final housekeeping point remained. The Raspbian root filesystem has an fstab that tries to mount / from the SD card. I needed to change that. So, I mounted the new USB hard drive under /mnt/raspbianroot and modified the file:

sudo mkdir /mnt/raspbianroot
sudo mount /dev/sdd1 /mnt/raspbianroot
sudo vi /mnt/raspbianroot/etc/fstab

I needed to change the following line:

/dev/mmcblk0p2  /               ext4    defaults,noatime  0       1

To this:

/dev/sda1  /               ext4    defaults,noatime  0       1

With that done, I unmounted the USB drive, and plugged both the 16 MB SD card and the USB hard drive into the Raspberry Pi (to power the drive, I had to use a powered USB hub, but you get the idea). The Raspberry Pi booted perfectly, and I verified that it was running from the USB hard drive by checking that /dev/sda1 was mounted at /.

Note: the new partition on the USB hard drive will be exactly the size as the partition on the 4 GB SD card. Since I wanted to extend the partition to take up the full drive, I ran fdisk and printed out the current partition information, and made note of the starting block. Then, I deleted the partition, and recreated it using the same starting block. I made sure the partition spanned the entire drive. I wrote the table out, rebooted, and then ran resize2fs to resize the filesystem.

Wrapping Up

Making the Raspberry Pi boot from a 16 MB SD card was pretty easy. Essentially it involves cloning the boot partition to the smaller SD card and the root partition to some other USB storage medium. As long as the boot partition continues to be smaller than 16 MB, this trick will continue to work.