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I want to backup my running Raspberry Pi to an SD card which is connected via an USB card reader.

As far as I understand, the device name of the onboard SD card in /dev is

brw-rw---T 1 root 179,   0 Jan  1  1970 mmcblk0

But how can I find the correct device name of the SD card which is conncted through the USB card reader? The LED of the card reader is on. I guess that this is good starting point.

Once I know the exact device names I know how to create a bakup usind dd. But since dd is also nicknamed destroy disk, I want to make sure I'm using the right device names before I start.

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  • 1) Make sure their is a card in your card reader and then plug it into the Raspberry Pi. If using a powered HUB then the Raspberry Pi will not reboot
  • 2) Then run "dmesg | tail" and look for /dev/sdX where X is a, b, or c (for example, the below uses 'a').
  • 3) Now as long as the plugged-in card is the same size (or larger) than what is in the Raspberry Pi SD card slot, you can run "dd if=/dev/mmcblk0 of=/dev/sda bs=1b"
  • NOTE: /dev/mmcblk0 refers to the whole SD card and not any one partition
  • NOTE: /dev/sda refers to the whole SD card in the external reader and not any one partition
  • 4) This will take anywhere from 10 min to an hour depending on the size of the SD card you have. When it's done you should as a minimum run "fsck /dev/sda2" since you copied a running "live" filesystem.

Not only is this a backup of your whole SD card (W95 "/boot", and Linux ext4 "/"), it can also be used to boot the system.

PS: I've also used this to copy my bootable 4 GB SD card to a 16 GB card and after using fdisk to make an ext4 on the remaining 12 GB and mount as /opt1.

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I don't have a card reader to try with, but I would first do:

  pi@raspberrypi ~ $ ls -al /dev | grep mmc
  brw-rw---T  1 root floppy  179,   0 Dec 31  1969 mmcblk0
  brw-rw---T  1 root floppy  179,   1 Dec 31  1969 mmcblk0p1
  brw-rw---T  1 root floppy  179,   2 Nov 29 22:13 mmcblk0p2
  lrwxrwxrwx  1 root root           9 Nov 29 22:13 root -> mmcblk0p2
   pi@raspberrypi ~ $ pi

The SD card is on /dev/mmcblk0 . This is also verified by looking at /etc/fstab which mounts the file systems at boot.

  pi@raspberrypi ~ $ cat /etc/fstab
  proc            /proc           proc    defaults          0       0
  /dev/mmcblk0p1  /boot           vfat    defaults          0       2
  /dev/mmcblk0p2  /               ext4    defaults,noatime  0       1
  pi@raspberrypi ~ $ 

Remove the card reader and look at the device file system and see what disappears. Put the card reader back in and see which device reappears.


pi@raspberrypi ~ $ cd
pi@raspberrypi ~ $ ls /dev > before.txt
pi@raspberrypi ~ $ #plug in the usb reader with the SD card
pi@raspberrypi ~ $ ls /dev > after.txt
pi@raspberrypi ~ $ diff -C 2 before.txt after.txt 
*** before.txt  2012-12-01 13:29:58.234244009 -0500
--- after.txt   2012-12-01 13:31:04.313366798 -0500
*** 56,59 ****
--- 56,62 ----
+ sda
+ sda1
+ sda2
*** 134,137 ****
--- 137,141 ----
+ usbdev1.4
pi@raspberrypi ~ $ lsusb
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 002: ID 0424:9512 Standard Microsystems Corp. 
Bus 001 Device 003: ID 0424:ec00 Standard Microsystems Corp. 
Bus 001 Device 004: ID 058f:6366 Alcor Micro Corp. Multi Flash Reader
pi@raspberrypi ~ $

This is showing that usbdev1.4 was added - the card reader. Also /dev/sda /dev/sda1 /dev/sda2 .

This is a second RPI card (not the running one) in the reader so it has two partitions mounted from /dev/sda1 and /dev/sda2 .

pi@raspberrypi ~ $ df -k
Filesystem     1K-blocks    Used Available Use% Mounted on
rootfs           1804128 1685924     26556  99% /
/dev/root        1804128 1685924     26556  99% /
devtmpfs          224436       0    224436   0% /dev
tmpfs              44900     228     44672   1% /run
tmpfs               5120       0      5120   0% /run/lock
tmpfs              89780       0     89780   0% /run/shm
/dev/mmcblk0p1     57288   16896     40392  30% /boot
/dev/sda1          57288   16920     40368  30% /media/8B12-9112
/dev/sda2        1804128 1256004    456476  74% /media/29b6c2f5-5469-49f2-abd5-daa9149021cc
pi@raspberrypi ~ $

The mount points for the partitions show up on the right.

pi@raspberrypi ~ $ cd /media/8B12-9112/
pi@raspberrypi /media/8B12-9112 $ ls
bootcode.bin  fixup_cd.dat  kernel_cutdown.img    start_cd.elf
cmdline.txt   fixup.dat     kernel_emergency.img  start.elf
config.txt    issue.txt     kernel.img
pi@raspberrypi /media/8B12-9112 $
share|improve this answer
Thanks for explaining how to find the device of the onboard card. Unfortunately I have no clue what look at the device file system means. – BetaRide Dec 1 '12 at 14:26
linux makes many hardware devices look like files in the file system. These devices show up in the /dev directory. – Dan M Dec 1 '12 at 16:52

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