I hadn’t connected my ipod classic on my computer for 6 months. Recently I wanted to sync it. It appeared in finder but not in itunes.
I tried on others computers and usb ports : no success.
I checked iPod with Diagnostics Mode (see turingtest2 answer on Apple Support Communities )
- Hold Center button + Menu button for 6 seconds.
- As Apple logo appears, immediately hold Center button + Left button 6 seconds
- Release all buttons as Diagnostic Mode Boots
- Go to : Manual Test > IO > HardDrive > HDSMARTData
Everything seems OK.
Since I didn’t care to lose ipod data as everything is on my mac, the solution was simply to erase the ipod with Disk Utility.
When erase was done, iTunes showed my brand new iPod :

