When iPod was first introduced, and up until late 2006 or so, there was a standalone utility that ran checked for and installed software and firmware updates for iPods. The iPod Update Utility was a simple item that was saved in your Utilities folder and automatically launched when anything new was available. One great feature of this simple updater is that it archived each update so subsequent updates to other iPods did not need to be re-download.
Nowadays, iPod updates come directly through iTunes. If there’s anything new when you plug in your iPod, iTunes will alert you and ask whether you’d like to apply them. Because part of my job is to rest and refurbish used iPods, I find myself needing to perform these updates all the time; our rural location also means we don’t have the fastest internet access, so not having to re-download updates is a boon.
While the iPod Update Utility stored the update files in plain view, iTunes tucks them away in your User’s library folder at ~/Library/iTunes/iPod Software Update. This file is only accessible to the user currently logged in, so if you have multiple users on your computer, each update will download once per user.
As the iPod operating system (commonly called iPhone OS) grows more robust in features and stability, it also grows considerably in size. Under the new paradigm, the old versions of the software are not updated and replaced, but a new version is added to the User’s folder at the cost of 225MB or more per update. Weeding out older updates will help in conserving hard drive space when you come upon the ceiling of storage for your machine.
Another thing to consider if you have an iPod that fails to update is that maybe it is one of the updater files that is corrupted. If your iPod seems dead and you can’t restore it, try removing the updater files from ~/Library/iTunes/iPod Software Update. Restart iTunes and let the updates come down again, and you may just be lucky enough to get that iPod up and running again.