Best way to do a server backup

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http://serverfault.com – I have a client who provides a data backup utility. He would like to make backups of the data users upload on a separate server/location. What's the best way to go about doing this? Its not about how quickly to come back online its just about not losing data. There will be a a huge amount of data we are talking 5+ GB a user with a lot of users. Sorry i cannot be more specific than a lot because he has not provided that info. I am not sure of the operating system but can guess its some flavor of linux. Ideally there would be no data loss and should be as up to date as possible Budget also (HowTos)