I work in two PCs and I sync all my files from my primary PC to a USB flash memory. In my second PC, I mount the USB flash memory to the same path to work on my files as I were on my primary PC. Now for the sake of performance and flash memory lifetime, I need to use any type of differencing, aka overlay, aka union, file-system (like unionfs or aufs) to let me use the USB flash disk as read-only and write changes to a temp directory and at the end allows me to write the changes back to the USB flash at once. Any help? Any hope?