I don't have an external drive but if I did I wouldn't treat it any differently to an internal drive. I would first check to see if it need defragmentation, I do the same for may second internal HDD and that is primarily for backups (images of the primary HDD partitions, and other data files, etc.).
If it needs defragmentation then it gets done, but it will frequently take much longer than my primary HDD because the image backups are around 2GB for each one (2) and it takes about 30 minutes for one of these to be defragmented. So if it were left for any length of time it would be a massive task, in fact after doing an image backup, I check and if needs be it will get a defrag right after the backup.