Zend certified PHP/Magento developer

How to sync files not directories in Windows

I have thousands of files of a certain extension spread through dozens of folders.
I routinely copy the files to a USB without the folder structure using a batch file.
To do this I have to wipe the USB each time before running the batch file.
I would like to be able to keep the two synchronized to eliminate the wiping.
I cannot find a tool that does such a sync. FreeFileSync doesn’t. Robocopy has a mirror function, but that would probably mirror the folder structure.
Is there a way to synchronize files of a certain extension but not the folders?
Thanks in advance.