Create a disk image of an sdcard

First, insert the sdcard and confirm it's recognized by your system. Determine the kernel device assignment (sdX in this example). To create a compressed disk image from an sdcard, use the dd utility.

$ sudo dd if=/dev/sdX | gzip -c > /path/to/sdcard.img.gz

To extract the compressed disk image onto an sdcard using the reverse process

$ gunzip -c /path/to/sdcard.img.gz | sudo dd of=/dev/sdX