You need to authorize and deauthorize the computers appropriately for using the different ipods on different machines.
Read this article from apple web site.
Deauthorizing a computer allows you to manage which computers can
play music, videos, audiobooks, or other content purchased from the
Authorization helps protect the copyrights on the content you buy.
You can use your digital rights management (DRM)-protected music
purchases1 from the iTunes Store on up to five different
computers (these can be any mix of Macintosh or Windows-compatible
computers). When you play an item you've purchased, your computer is
"authorized" to play content purchased using your Apple Account.
: Songs you encode in AAC format from a source
other than the iTunes Store, such as your own audio CDs, and songs from
iTunes Plus downloads do not need to be authorized.
To authorize a computer to play purchases using your Apple Account
- Select a song, video, or audiobook you've purchased (in your library or the Purchased playlist).
- Click the Play button.
If the computer is already authorized, the item plays. If the
computer has not yet been authorized, iTunes asks you to enter the
Apple ID and password you used to purchase the song.
Remember to deauthorize your computer before you sell it, give it away, or get your computer serviced.
To deauthorize a computer
- Open iTunes.
- Choose Store > Deauthorize Computer (In earlier versions of iTunes, access this option from the Advanced menu).
- Select "Deauthorize Computer for Apple Account" and enter your Apple ID and password.
Make sure you deauthorize your computer before you upgrade your RAM,
hard disk or other system components, or reinstall Windows. If you do
not deauthorize your computer before you upgrade these components, one
computer may use multiple authorizations.
To deauthorize all computers associated with your account
If you find you have reached 5 authorizations, you can reset your
authorization count by clicking Deauthorize All in the Account
- Click iTunes Store in the menu on the left side of iTunes.
- If you're not signed in to the store, click the Account button, then enter your account name and password.
- Click the Account button again (your ID appears on the button), enter your password, and then click View Account.
- In the Account Information window, click Deauthorize All.
: You may only use this feature once per year.
The Deauthorize All button will not appear if you have fewer than 5
authorized computers, or if you have used this option within the last
12 months. If you need assistance on using this feature, please contact
the iTunes Store support via email (http://www.apple.com/support/itunes/ww/
- iTunes DRM-protected music includes audio with a bit rate
of 128 kbps and allows users to transfer songs and videos to up to five
computers, burn seven copies of the same playlist to CD, and sync to an
unlimited number of iPods.
- iTunes Plus
music refers to songs and music videos available in our highest-quality
256 kbps AAC encoding (twice the current bit rate of 128 kbps), and
without digital rights management (DRM). There are no burn limits and
iTunes Plus music will play on all iPods, Mac or Windows computers,
Apple TVs, and many other digital music players.
- While you
may need to enter your account information again after initializing or
formatting your hard disk, initializing or formatting the hard disk
itself does not remove the computer from the list of authorized
computers. If you plan to initialize or format your hard disk prior to
selling or donating your computer, deauthorize the computer first, then
initialize or format the hard disk.
- Under some
circumstances you may be required to enter your registration
information again, but this should not use up multiple authorizations.
If you think that you may have used up multiple authorizations on just
one computer, see One computer using multiple iTunes Store authorizations.
- The iTunes Store is governed by its Terms of Service, which permits Apple to change the authorization policy.
- You can authorize computers not located in the United States so you can play previously purchased music.
Apple ID based on an expired .Mac (pronounced "dot Mac") or Mobile Me
account name and password can still be used to purchase items and to
authorize or deauthorize computers to play purchased music.
See the following documents for additional information:
HT1373: "iTunes: How to Copy Music Between Authorized Computers
HT1727: "iTunes for Windows: How to Copy Music Between Authorized Computers
HT2204: "iTunes: About Apple ID and Password