[all variants] Freeware Backup recommendations

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http://ubuntuforums.org – I want a backup application that backs up single files and folders (not the system). I already have an app for that and I like to keep the system and files separately (like an image and file backups). I would prefer it work with both Windows 7 and Linux. I have already tried Nero BackItUp, Windows 7 built in backup, and Western Digital Sync Backup. But they all give errors. What do you recommend that will work. I thought of Carbonite, but I already have an external drive. So I shouldn't really be paying a yearly fee. (Hardware)