How to clone linux installation onto an external harddisk?

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http://unix.stackexchange.com – I see a lot of dd related questions, but could not find answers to the following. If question/answers already exist, please point to them, then flag to close, delete or merge. Can a working linux installation (say on /dev/sda7) be cloned to an external hard disk partition (say /dev/sdb2), while on it? (/dev/sdb is a 1TB USB hard disk, /dev/sdb2 is of size 100GB) If not, can I do it using a live cd? Will the cloned partition on external drive be bootable? If it is not bootable, how to do that? Why want to do that ? : Need to carry the cloned external drive than the laptop itself, with all (HowTos)