If you have Ubuntu installed on a single harddisk and you have another harddisk of equal or larger storage capacity, then cloning the disk is really simple:
sudo dd if=/<from disk> of=<to disk>
So, for example, if you want to clone sda to sdb, then you would do
sudo dd if=/dev/sda of=/dev/sdb. That would retain all partitions, boot manager, etc.
Be careful to use the correct if (input file) and of (output file). The input file is the disk you want to copy. If you switch order, then you will clone the empty disk onto the full one, which will leave you with two blank disks. :)
Also make sure that the new disk has sufficient space to clone the first disk. Otherwise it'll just be chopped off at the end. There will be no warnings.
If you want to create an image, then simply replace
of=/some_place/disk-image.raw. Store it on another disk.