USBFlahCopy automatically backs USB flash drives' content by running in the background.

As useful and handy as they are, USB flash drives are not a magic wand: they can be lost, damaged, or have their contents accidentally deleted. A solution against those risks is to constantly back up their contents as you go. But that's where lies the challenge for most users: remember to do it faithfully. The good news is that there is a new app called USBFlashCopy that will automatically back up the entire content of your USB flash drives every time it is connected to your PC.
When installed in a PC, the app runs quietly in the background until a USB flash drive is plugged in and the suite will jump into action and start the drive's back up before you even can start using it. The app is a smart one who does differential backups only, meaning only files recently added or changed will be backed. The app takes very few storage space and doesn't require a full installation and is transferable to another PC or accessed from afar with a network drive or any other location according to AppScout.com.
The app is free for non commercial users and business users can buy a license for $39.99.The app works with Windows 2000, XP, Vista, and Windows 7, in both 32 and 64-bit versions.

For more, see AppScout.com