WD has a knowledge base article, Leaving an external hard drive powered on all the time, that states:
Turning on the drive a few times per day is considered normal usage and should not pose any problems. If a drive is turned on and off excessively on a daily basis, this could affect the longevity of the hard drive’s components.
I just purchased two WD external drives (model WD Passport Mac). Both had auto-sleep set to a 10-minute default. This seems to qualify as turning the drive on and off “excessively”. I’ve changed it using the WD Drive Utilities application. But I’m curious why WD would set a default sleep time on its drives that violates its own guidance as to how to avoid excess wear-and-tear on the drive.