The dd utility can be successfully used to create a backup of a filesystem (in lack of more specialised backup utilites).
dd if=/dev/sdaX | gzip > /backup/sdaX.gz
where sdaX is the partition you want to backup, and the /backup folder is located on a different disk/partition than the one you’re backing up..
Only note that dd does not care whether a sector is used or not, it will copy it to the image file nonetheless. Zeroing out all free space before backup up is a good way to keep wasted backup space to a minimum (since the backup will be compressed)