How can I check the iPod for disk errors or problems? How do I "defragment" the hard drive?

To scan for disk errors, you will need to first reset the iPod.

For the original iPod, iPod (2nd Gen), and iPod (3rd Gen) plug the iPod into the power adapter and plug the power adapter into an electrical outlet (or plug the iPod into a computer), and reset it by depressing the "menu" and "play/pause" simultaneously and continue to hold both buttons until the Apple logo appears.

For all models with a "Clickwheel" -- starting with the iPod (4th Gen) and provided by all subsequent models with the exception of the iPod shuffle and iPod touch lines and the iPod nano (6th Gen) -- plug the iPod to the power adapter and plug the power adapter into an electrical outlet (or plug the iPod into a computer), and reset it by depressing the "menu" and "select" (the center button) until the Apple logo appears. For additional details on resetting the iPod, please refer to "Resetting iPod" from the Apple Support Site.

When the Apple logo appears, release the buttons and press "rewind", "fast forward", "menu" and "action" (the center button) simultaneously. It can be tricky to press all these buttons at the same time. You may find it helpful to place the iPod on a desk or other flat surface and use both hands to press the required buttons.

For the iPod shuffle models, move the switch to the "off" position, wait for five seconds, and switch it back on. For details, please refer to "Resetting iPod shuffle" from the Apple Support Site.

For the iPod touch models, "press and hold the sleep/wake button and the home button at the same time for at least ten seconds until the Apple logo appears". For more information, please refer to "Turning on/off and resetting iPod touch and iPhone" from the Apple Support Site.

For the iPod nano (6th Gen), "press the Sleep/Wake button and the Volume Down button for 6 seconds, until the Apple logo appears."

When the correct combination of buttons is depressed, the iPod will scan its storage for errors. For iPods with displays, if a checkmark icon appears, then the test passed and no errors were found. If the iPod shows an "X" or an icon of an iPod with a "frowning face", then the test failed and repairs are needed.

Apple recommends that one not use third-party software to defragment the hard drive on iPod models with hard drives, and instead use the "restore" function. Before using the "restore" function, be very certain that all music and other data on the iPod is backed up, as restoring the iPod will completely erase the hard drive. For additional information on restoring the iPod, please refer to "Restoring iPod to Factory Settings" and "Updating and restoring iPhone and iPod touch" each from the Apple Support Site.

What is the difference between "update" and "restore"?

According to Apple, "update simply updates the software that controls iPod and may also update the firmware for the hard drive. Update does not affect the songs and files stored on iPod's hard disk." Whereas, "restore erases the hard disk and restores iPod to its original factory condition. Because restoring iPod erases all of the songs and files on its hard disk, make sure to back up your songs and files first."

For additional information regarding updating and restoring your iPod, please refer to "Updating iPod Software" and "Restoring iPod to Factory Settings", respectively.

For information on updating and restoring your iPod shuffle, please refer to "Resetting iPod shuffle" and "Restoring iPod shuffle to Factory Settings", respectively.

For the iPod touch, please refer to "Turning on/off and resetting iPod touch and iPhone" and "Updating and restoring iPhone and iPod touch." All of these documents are from the Apple Support Site.