From Blue Integer:
This freeware utility prevents hard drives that are connected to USB docking stations from powering down (because of their firmware) due to inactivity of the read/writes heads. It simply writes to a log file at regular intervals thus preventing the frequent power up and down, consequently extending the lifespan of the hard drive.
Just extract the contents of the .zip file to a location of the external hard disk and then run "Hard Disk No Stop.exe". The hard drive will stay active as long as the program is running. Double clicking on the tray icon will restore the program's window. Also, by right clicking the tray icon you can exit and restore the program as well.