It sounds like you are connecting your iPod Classic to your computer and the screen on the iPod says Do Not Disconnect but it is not seen in iTunes. This device appears to be in Disk Mode. Take a look at the info in the folowing article:
About the Do not disconnect and OK to disconnect screens for iPod
"Do not disconnect" screen.
As long as iPod is connected to the computer, and you have not ejected it from the Finder or iTunes, the "Do not disconnect" screen appears.
To disconnect iPod from your computer, you must first eject it. Follow the instructions in "Disconnecting iPod from your computer". Once you have ejected iPod, the second disconnect screen appears.
"OK to disconnect" screen
You may now safely unplug the FireWire or USB cable from iPod.
- When you first connect iPod to your computer, the "OK to disconnect" screen may briefly appear until the computer recognizes iPod is connected. This is normal.
- These screens do not appear if your computer is in sleep mode. Wake your computer before disconnecting iPod.
- These screens do not appear if iPod is connected to the Apple iPod Power Adapter. You may disconnect iPod from the Apple iPod Power Adapter at any time.
To avoid this behavior in the future, make sure that you safely disconnect your iPod prior to disconnecting it from your computer.
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All the very best,