How can I use the iPod as a hard disk?

To use the iPod as a hard drive, you will need to "enable disk use" in iTunes and connect the iPod to your computer (please note that the iPod touch models do not support disk mode). When disk use is enabled, the iPod appears as any other disk on the Mac desktop or within My Computer on Windows, and you can drag-and-drop files from the hard drive to the iPod or vice versa.

For additional information, please refer to "Using Your iPod as a Storage Drive" from the Apple Support Site.

Can I start up my Mac or Windows computer using the iPod?

You can use the iPod to start up a Mac, although the official information provided by Apple is a bit contradictory as to whether or not it is a good idea.

The official Apple iPod FAQ states:

iPod can be loaded with system software and used as a startup disk for your Mac or to install system software on your Mac. Using iPod to start up your Mac with system software is an unsupported feature. iPod cannot be used to start up a PC.

However, "unsupported feature" links to another document titled "Don't Use iPod as a Startup Disk" that simply states "though your computer might recognize iPod as a possible startup disk, Apple does not support using it as a startup disk. You can store all kinds of files on iPod, even system software, but you shouldn't use iPod as a startup disk."

The official Apple iPod FAQ is referring to now ancient Firewire-based iPods. USB-based iPods, which include all currently shipping models (and many older ones as well), still are not officially intended for use as a Mac OS X startup disk even as an "unsupported feature."

Apple previously had a document in the Apple Support Site entitled "Don't Start Up from an External USB Device," but has subsequently changed this document to provide instructions for "Starting from an external USB storage device (Intel-based Macs)."

Ultimately, many iPod models will function as a Mac OS X startup disk, which is extremely useful for troubleshooting. Apple most likely formally recommends against it, however, as the iPod could overheat.

Also see: Which iPods can sync and charge via USB? Which iPods can sync and charge via Firewire?

What operating system do the iPod models use? Do they run Mac OS X?

According to San Francisco Chronicle article "Little-known startup was behind iPod's easy-to-use interface", all versions of the iPod -- except for the iPod shuffle -- released prior to September 5, 2007 use an operating system derived from Pixo OS 2.1, originally designed for use with cell phones and other handheld devices.

The iPod models introduced on September 5, 2007 -- the iPod classic (6th Gen/Original), iPod nano (3rd Gen/Fat), and iPod touch (Original) -- as well as subsequent models use operating systems derived from the version of "OS X" used on the iPhone, although please note that only the iPod touch, iPhone and iPad models are capable of running third-party applications (other than a small selection of third-party games that were available for some earlier iPod models).

On March 17, 2009, upon unveiling a developer's preview of the third version of this "mystery" operating system, Apple started referring to it as the "iPhone OS" and on June 7, 2010, Apple changed the name again to simply "iOS".

Also see: Maximum Supported Version of iOS from the "By Capability" section of Everyi.com.