How to move rootfs to external usb storage

This instruction is valid for linux OS.

Prepare raspbian image
Download wheezy-raspbian image from official site.

http://www.raspberrypi.org/downloads

Unzip downloaded file and put img file under working directory.
Also create following directory for mount target.

• mnt_src_rootfs
• mnt_dist_rootfs

Flash image to sdcard
We don't need rootfs on sdcard finally. But it is useful to check device entry for your external usb storage in this instruction.

# sudo dd bs=1M if=2014-01-07-wheezy-raspbian.img of=/dev/sdc

Mount partitions in image
Check wheezy-raspbian image file with fdisk.

# fdisk -l -u 2014-01-07-wheezy-raspbian.img

Disk 2014-01-07-wheezy-raspbian.img: 2962 MB, 2962227200 bytes
255 heads, 63 sectors/track, 360 cylinders, total 5785600 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: 0x000981cb
 
Device Boot Start End Blocks Id System
2014-01-07-wheezy-raspbian.img1 8192 122879 57344 c W95 FAT32 (LBA)
2014-01-07-wheezy-raspbian.img2 122880 5785599 2831360 83 Linux
 
Mount 2nd partition with offset.

# sudo mount -o loop,offset=$((122880*512)) 2014-01-07-wheezy-raspbian.img mnt_src_rootfs

Prepare external usb storage
Create ext4 partitions on your extarnal usb storage by fdisk and mkfs.
And mount it as 'mnt_dist_rootfs'.

Copy rootfs
Copy all rootfs files from img file to extarnal usb storage.

# sudo cp -ax mnt_src_rootfs/. mnt_dist_rootfs

Boot Raspberry pi
Insert sdcard and bootup. (Here, rootfs on sdcard is used.)
On raspbian,

1. login
2. connect usb extarnal usb storage.
3. execute dmesg
4. Get device entry for the ext4 partition on extarnal usb storage

I will assume your device entry is '/dev/sdc1'

Modify cmdline.txt

On raspbian,

# nano /boot/cmdline.txt

Modify following path to device entry of rootfs partition on extarnal usb storage.

root=/dev/sdc1

Sync and reboot rasbian.

# sync

# reboot 0

That's it.