The primary purpose of RealtimeSync is to execute a command line each time it detects changes in one of the monitored directories or when a directory becomes available (e. g. insert of a USB-stick). Usually this command line will trigger a FreeFileSync batch job.
Start RealtimeSync.exe located in FreeFileSync's installation directory and enter all folders you want to monitor. Instead of doing this manually you can import a ffs_batch file via Menu → File → Open. This not only extracts all directories relevant for synchronization but also sets up the command line to execute the ffs_batch file each time changes are detected. Now press Start to begin monitoring.
Save a ffs_batch configuration in the USB stick's root directory,
e.g. H:\ and let FreeFileSync run it when the stick is mounted.
But instead of hard coding the USB drive letter H:\ (which may change occasionally),
refer to the USB stick via its volume name instead.
Configure RealtimeSync as follows:
Whenever directory H:\Data becomes available, RealtimeSync executes the command line which starts the batch job located on the stick. RealtimeSync will also trigger each time files are modified in H:\Data.
The command line usually starts a synchronization task using FreeFileSync which naturally leads to additional file change notifications. Therefore the RealtimeSync change detection has to be deactivated to not go into an endless loop. On the other hand it is not likely that changes (other than those from FreeFileSync) happen in first place since RealtimeSync runs the command line only after a user-specified idle time has passed. This makes sure the monitored folders are not in heavy use. In any case, files changed during the execution of FreeFileSync will be synchronized the next time FreeFileSync runs.