Tools needed:
sudo apt-get install inotify-tools
Example script:
inotifywait -mrq -e create -e modify -e delete -e attrib -e move ~/Maildir | while read file do (notify-send "File updated in Maildir:" "$file"&) done
The script will watch the Maildir directory for file changes and send a notification to the desktop.
To get all events just omit the event list:
inotifywait -r -m /media/usbdisk/