Создание архивированного образа карты

Pretty good and simple way to deal with this is simply pipe it via gzip, something like this:

# dd if=/dev/sdb | gzip > backup.img.gz

This way your image will be compressed and most likely unused space will be squeezed to almost nothing.

You would use this to restore such image back:

# cat backup.img.gz | gunzip | dd of=/dev/sdb