Manage devices only allows me to remove and not to add which is grayed out.

Authorising a computer is to allow your new iTunes Library to download music purchased from the iTunes Store, play it on your computer and copy it to an iPod. You shouldn't need to authorise an iPod Classic.

However, when you connect the iPod to your new computer, you see a message telling you that if you Sync this iPod with your replacement iTunes Library then all the music on the iPod will be deleted and replaced with only what is in your new Library.

So there are two things you need to do, or confirm:

1. 1. if your iPod is not seen in your new Library, you need to fix that issue. If your iTunes does not see the iPod, go to If your computer doesn’t recognise your iPhone, iPad or iPod – Apple Support (UK). It should take you to your country's page.
   2. ensure that your new Library has your music in it before attempting to Sync the iPod with it

If the music is not in your Library, the remedy depends on where the music came from originally:

• • if your music is from the iTunes Store, sign into the store in your new iTunes and then download your purchases. This may be when (and why) you need to authorise the new computer. See the article you came from (above, at the top of the page). Note that if you have too many computers authorised, or the same computer authorised several times, then you will need to de-authorise at least one in order for the new one to be authorised
  • if your music is from other sources, do you have a backup of that music? If so, copy it onto your new computer, add it to your new iTunes Library. If you do not have a backup, you will need to go back to your original sources for it:
    • from CD - re-copy from the CD
    • other online stores - go to your account with that store and re-download

See Authorization vs. association - Apple Community. Association is caused by using certain features, such as downloading past purchases on a computer or iOS device. It doesn't apply to iPod classic.

See Recover media from an iPod - Apple Community to copy the content of your device into the library.

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