Edit: it turns out SyncBack SE has a trigger action:
Open Profile, go to “When” tab, then
“Insert”. It lets you specify by drive
letter, label, or serial number.
However the SE version is not free and Windows only (I think)
Or you could try adding an Autorun event for your syncing program (example for SyncToy) using TweakUI
I can't seem to find any (other) program that will start automatically when you connect your USB device.
They all rely on scheduling to start syncing. You could decide to have it scheduled every our, it will simply fail if the USB device is not connected and run if it is.
But too be honest it would be easier if you simply clicked on the sync button, you have to plug in your USB device manually as well.
Anyway I also found a synchronization tool that runs on both Linux and Windows: DirSync Pro which is completely free, runs on Java and has a nice GUI:
o Powerful synchronization algorithm.
o Bidirectional (Two way) and Unidirectional (One way) synchronization mode.
o Option for various behavior of conflict resolution for Bidirectional Synchronization.
o Synchronizes unlimited number of folders.
o Large number of options to change the synchronization behavior.
o Option to synchonise subdirectories recursively.
o Synchronizes files/folders any file system (FAT, FAT16, FAT32, NTFS, WinFS, UDF, Ext2, Ext3, ...).
o Synchronizes files from/to network drives
o Synchronizes files from/to any mounted devices (Harddisks, USB-Sticks, Memory cards, External drives, CD/DVD's, ...).
o Synchronization could be used for making incremental backups.
o Option to create up to 50 backups from the modified/changed files before synchronization.
o Option to define a backup folder.
o Option for handling symbolic links.
o Option for handling time-stamps.
o Easy, clear and user-friendly graphical user interface, no unnecessary gadget you never use.
o Runs on every modern operating system including Windows™, Linux™ and Macintosh™
o It is Portable! It does not need any installation. Just run the application!
o Open source, it is 100% free of charge, 100% free of commercial text, 100% free of advertisements and 100% free of spyware.
o No time/function limitations
o Uses no local database, so no overhead
o Does not need any installation. Just download and run it. You can put it on you USB-stick en you can run it on any computer/any platform.
o Advanced logging/reporting facilities. Just select a log level and define where to write the log.
o Option to log on application level (default log)
o Option to log on each directory level (dir log)
o Option to define the log leven (how much to log)