Q: cant backup ipod classic 160 gb in itunes

The backup function in iTunes is only for iOS devices, like iPhone and iPod touch.  There is no backup function for media player iPods (like iPod classic).  All of the songs (and other media) on your iPod should exist in your iTunes library, because all of it was synced from your iTunes library.  Your iTunes library is your iPod's "back up."

What you need to back up is your iTunes library (on the computer), in case your computer (or its drive) fails.  This Apple document describes one way to back up your complete iTunes library to an external drive.

Manage and back up your iTunes media library - Apple Support

See Recover your iTunes library from your iPod or iOS device if the iPod is the only device holding some of your media.

See Backup your iTunes for Windows library with SyncToy for an effective backup strategy for iTunes.

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