USBsyncer is an automatic synchronization tool that synchronizes a folder on your hard drive with a folder on your removable drive when you insert a USB drive into your PC. So you always have the most recent files on your drive without having to copy them by yourself. This is useful, for example, if you are using a portable Firefox browser and want to synchronize your local bookmarks with it; you jut point USBsyncer to your Firefox profile folder and let it copy it to the profile folder on your external device.

However currently, only one-way synchronization is possible with this tool. This means that it just copies the contents of the hard disk folder to the folder on the removable drive, so changes in the folder on the drive will be discarded.

After installation, right-click on the USBsyncer icon from your system tray and choose 'Configure drive'. This will help you to set up your USB stick for synchronization. Then, choose 'Synchronize now' to perform your first synchronization.
You can download USByncer free from here