Plug Micro SD card with reader or plug in USB flash into Linux box

fdisk -l
...
Disk /dev/sdd: 16.6 GB, 16574840832 bytes
...

Back it to file: 

dd if=/dev/sdd of=/root/yun-sd-card.bin

Linux dd Command Show Progress Copy Bar With Status:

http://www.cyberciti.biz/faq/linux-unix-dd-command-show-progress-while-coping/

Monitor Progress of Data Sent Via a Pipe:

http://www.cyberciti.biz/open-source/command-line-hacks/pv-command-examples/

Install pv:

apt-get install pv

Back up it to file:

15.4GB 0:15:03 [17.5MB/s] [==================================>] 100%
4046592+0 records in
4046592+0 records out
16574840832 bytes (17 GB) copied, 903.16 s, 18.4 MB/s

Restore from backup file to new Micro SD card:

pv -tpreb  /root/yun-sd-card.bin | dd of=/dev/sdd  bs=4096 conv=notrunc,noerror