Given this is Linux, dd has some issues with a live file system, that is issue 1, issue 2 is that dd is only accurate (usually) on a cold file system when attempting an image style backup, which means you have to take the root file system offline to ensure a consistent state of capture with dd, replicate to a different file system/disk from the backup source. I don't know of anything in the ARM implementation of Linux the Pi uses that addresses either issue as noted. I backup (most) of the live file system of an active/live Pi via rsync, to a Windows system share using cwrsync in fact, but rsync with the proper exclusions is a selective backup... so the restore is a bit of trick. You install a clean copy of the Linux OS, and then use the rsync backup as a poorman repo to do a selective replace/restore of files that may have changed over time from a clean install, as a point in time restore that overwrites the clean install files as noted, this works well for say capturing DHCP, BIND, TFTP and other services configuration files, say in the /etc/sysconfig directory (since I like to test Fedora on Pi, sick, I know!). Using rsync is not perfect, or transparent, nor a image-based backup like most of the SD card backup solutions are, but it does let you capture most incremental changes of a live Pi without powering down the Pi and pulling the SD card every single time you want to backup a Pi.