how do you sync an old ipod to a new itunes library?

If you use the Sync Music setting (automatic syncing), you cannot sync an iPod to a "second" iTunes library. By definition, iTunes can only keep your iPod "synchronized" with ONE iTunes library at a time.

If you want to associate the iPod with a new iTunes library, here's how to do it. This erases the iPod and replaces its existing content with content from the new iTunes library. Select the iPod in iTunes (click its device button on horizontal bar) to show its settings screen in iTunes window. Along the left side (in the sidebar), under Setting, click Music. To the right, the iPod's Music settings screen is shown.

NOTE: Any songs on the iPod that are not in your iTunes library will be lost. If this is a problem, please post back.

Checkmark the box for Sync Music. This turns ON automatic syncing and associates the iPod to the new iTunes library. Below that, if your iTunes music library can fit on the iPod, you can choose the option to sync Entire music library. Otherwise, choose the option to sync Selected playlists, artists, albums, and genres. Then, make your selections from the lists below. For example, you can maintain one or more playlists in your iTunes library with songs you want on the iPod, and select them under Playlists (on the iPod's Music settings screen). Click Apply to sync your iTunes music library (or selected portion of it) to the iPod.

Does it still appear when you connect it to the computer with the old iTunes library? Please provide more details about your situation... Is the old confuter still working and accessible, with its iTunes library intact? Are you moving to a new computer?

If possible, try a different USB docking cable to rule out faulty cable as the cause.

When you connect it, watch the iPod's screen. Does it say "Do not disconnect" or "Connected" or give other indication of being connected? Or does it only show the battery charging symbol? Or does to not react at all to being connected?

Does the iPod still operate properly (as an iPod) when disconnected?

Do you use a Mac or Windows PC? If Windows, what version?

If the iPod's screen says "Connected," that's good. That means there is a data connection between the iPod and your Mac.

Since the iPod does not operate properly when disconnected, the correct action is to put it into Disk Mode...

i have tried disk mode but that does not help.

Did you use this procedure to put it into Disk Mode?

Putting iPod into Disk Mode - Apple Support

Can you provide mode details to explain "but that does not help." What happens when you put the iPod into Disk Mode? Are you saying you cannot put it into Disk Mode? Or are you saying iTunes does not react when you connect the iPod in Disk Mode?

What should happen, when you connect the iPod in Disk Mode to the Mac (with iTunes running), is for iTunes to prompt you to do a Restore on the iPod with a message about "recovery." Doing the Restore erases the iPod, reinstalls its software, and sets it to default settings.

Try restarting the Mac, running iTunes, and connecting the iPod (in Disk Mode) directly to the Mac with all other USB devices disconnected. (You can leave standard UISB keyboard/mouse connected, if used.)