There are various reasons (performance, space, ..) why you want to move your root partition from your Raspberry sd card to an external device like an USB disk or USB stick. You can do it manually or use a small script to achieve this.
raspiSD2USB.py will detect all possible new partition candidates and you can select the one which should become the new external root partition. All the root partition data from the SD card will be copied. If for some reasons the old SD card root partition should be used again just rename /boot/cmdline.txt.sd to /boot/cmdline.txt.
In order to detect root partition candidates the partitions have to
1) be mounted
2) be formatted with ext4
3) has to be at least as big as the root partition or (with option --force) has to have more free space than the root partition uses
A typical dialog will look like
raspiSD2USB.py V0.2.1 2015-04-12/20:41:05 0ff0dfd RSD0002I --- Detected following partitions RSD0003I --- /dev/mmcblk0p1 - Size: 112.00 MB - Free: 97.53 MB - Mountpoint: /boot - Partitionstype: vfat - Partitiontable: None RSD0003I --- /dev/mmcblk0p2 - Size: 2.85 GB - Free: 221.95 MB - Mountpoint: / - Partitionstype: ext4 - Partitiontable: None RSD0003I --- /dev/mmcblk0p3 - Size: 804.00 MB - Free: NA - Mountpoint: None - Partitionstype: ext4 - Partitiontable: None RSD0003I --- /dev/sda1 - Size: 3.84 GB - Free: 3.50 GB - Mountpoint: /mnt - Partitionstype: ext4 - Partitiontable: msdos RSD0028I --- Skipping /dev/mmcblk0p1 - Partition located on SD card RSD0028I --- Skipping /dev/mmcblk0p2 - Partition located on SD card RSD0028I --- Skipping /dev/mmcblk0p3 - Partition located on SD card RSD0009I --- Target root partition candidates: /dev/sda1 RSD0011I --- Source root partition /dev/mmcblk0p2: Size: 2.85 GB Type: ext4 RSD0012I --- Testing partition /dev/sda1: Size: 3.84 GB Free space: 3.50 GB Type: ext4 RSD0005I --- Following partitions are eligible as new target root partition RSD0006I --- /dev/sda1 RSD0007I --- Enter partion: /dev/sda1 RSD0019I --- Partition /dev/mmcblk0p2 will be copied to partition /dev/sda1 copied and become new root partition RSD0020I --- Are you sure (y/N) ? J RSD0021I --- Copying rootpartition ... Please be patient tar: Removing leading `/' from member names tar: Write checkpoint 1000 ... tar: Write checkpoint 236000 tar: proc: implausibly old time stamp 1970-01-01 01:00:00 RSD0022I --- Updating /etc/fstab on /dev/sda1 RSD0023I --- Saving /boot/cmdline.txt on /dev/sda1 RSD0024I --- Updating /boot/cmdline.txt on /dev/sda1 RSD0025I --- Finished moving root partition from /dev/mmcblk0p2 to partition /dev/sda1
raspiSD2USB.py is available under the GPLv3 on github