If the drive is simply on — fully powered up and spinning, even while it’s not needed — you would be using excess energy that you don't necessarily need to be using, and putting needless wear on the drive bearings. The average lifespan for an external hard drive (any external HD), assuming no physical damage occurs, is around 3-5 years, depending on the make, model and conditions it is stored in. The lifespan of an external HD may be shortened if you have it plugged in all the time and constantly spinning and therefore always drawing power, but normally this isn't an issue in any noticeable way for the vast majority of people. So long as you're taking good care of the drive and remembering to power down your computer when it's not in use, this will keep your external HD in tip top shape for its lifespan. Shutting off your computer when it's not in use will protect your machine and the drive from power surges as well. Taking the normal precautions you do with any of your electronics should be sufficient.


Cheers,
Ember