Before installing a replacement battery in my creaky old iPod, I wiped the drive, which boosted the flagging battery life from four hours to more than eight – almost as good as new.
I had no idea why, but Stephen Clarke-Willson says he does:
Sounds reasonable, but it's also kinda fishy. What if you update your songs a lot, like I did? Too bad I'm too lazy to check into it.
Update: Evan Zimmerman writes, "Best bet is to use Diskwarrior to rebuild it once it's mounted on a Mac. Serves roughly the same purpose as defragging but doesn't require the drive be wiped out."
