I've volunteered at a few places where we see (literally) thousands of machines donated each year. The number of machines that come in with "dead" drives is astonishingly few. Esp when you consider there was *some* reason the donor opted to "discard" the machine.
Well, thinking only about power consumption (conservation), spinning a disk down *does* save power -- *if* it's going to *stay* spun down! So, in a laptop, etc. it's a viable way to extend battery life.
Problem comes when you use such a drive in an app that won't let the drive "idle".
I have several (disk-based) iPods that I've had no problems with (so far). The Zune came to me "failing" so I have no idea what sort of life it had before...