Is it safe to clone the current used disk

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http://askubuntu.com – I have a hard drive I am using and I need to clone it, but I can not turn the computer off to take the HDD out. I want to know how safe it is to clone a disk that is being used at the moment it is being cloned. What problems could I have the cloned copy? What problems could I have with the original disk? What alternatives can I use (if available) to clone a working (in use) disk? (HowTos)