(though when you start looking at how much it costs to power the thing, it's still not free, and at some point it might have been cheaper to replace it with something else.
I don't think it really works that way for mechanical hard drives. At least, taking a quick look at the drive pile and comparing a few, there's an old 15gb ide that consumes .3 amps each for both rails, vs a couple 1tb sata drives that use .5 - .75 for both. Unless you're talking about replacing a mechanical with an SSD I don't think power consumption is really an issue. And depending on how expensive that SSD is compared to a bargain bin old mech, it may be a while before you catch up, especially if the machine isn't on 24/7.
