Verify SD is not mounted
|WARNING:||If you are working with the SD card from your workstation, keep in mind that most Linux distributions will mount the partitions that they can as soon as the drive is inserted. This is desirable if you want to open the filesystem, but for dealing directly with the block device for performing backups or restoring an image this is dangerous to your data.|
To verify if your workstation has mounted the block device on insertion:
cat /proc/mounts # look for your SD card block device to see if this is already mounted.
If the block device did automatically mount, you will need to refer to your distribution's documentation for disabling automounting. For example, this is Ubuntu's documentation on disabling automounting. If you are not using a graphical Linux system this should not be a concern, but make sure no filesystems are mounted read only or read write while writing or reading an image.
For backing up or restoring any images from the board you will need to make sure you do not have any partitions mounted. On the default configuration you can write an image from the initrd after unmounting /mnt/root: