I’m using a program on my flash drive which synchronises added files.
Comodo Backup does this much better but obviously is installed on my PC.
My suggestion for software would be Comodo Backup Lite.
This would be installed on the USB drive and would include password protection for the drive and perhaps encryption.
A useful feature could be a facility to include one file that isn’t encrypted, this could include details of the owner for returning the drive should it be lost.