If you want to use your iPod as an external disk, you first have to go into iTunes to configure this type of usage. To start, launch iTunes and connect your iPod to your computer. Once it shows up in the Devices section on the left side of the iTunes screen, select your iPod:

Select your iPod in the Devices section

Once your iPod is selected, you'll see Options in the right section of the screen. To be able to use your iPod as an external disk, you need to check the box next to Enable disk use.

Check "Enable disk use" to use your iPod as an external disk

Once you check this box, you'll get a warning like so:

Warning message when selecting disk use for an iPod

Go ahead and click OK. What this warning means is that instead of ejecting your disk in iTunes, you now need to eject your disk using Finder (for a Mac) or Windows Explorer (if you are on a PC).

Once you accept this, your disk will still not show up. Before your iPod will show up as a disk in the Finder, you need to hit the Apply button:

The last step in configuring your iPod as an external disk.

Once you do this, your iPod will be configured for use as an external disk and it will show up in the Finder. An important thing to make note of is that when using your iPod as an external disk you will not be able to see your music files. And if you transfer music onto your iPod using the Finder, it won't actually show up as music you can play on your iPod. All of the music controls are handled through iTunes.