Q: iPod Classic disconnecting as soon as I connect it

Previously (before "this morning"), were you able to connect the iPod and sync content with iTunes on this computer?  Currently, does the iPod still appear in iTunes when connected, before unexpectedly disconnecting?

First, as a test, shut down (power off) your computer.  Disconnect all USB devices; you can leave standard USB keyboard/mouse connected, if used.  Do this to Reset (restart) the iPod

Learn how to reset your iPod - Apple Support

Start up computer, run iTunes, and connect iPod to a direct USB port on the computer, by itself.  Is there any difference?

If that does not help, since it no longer appears in iTunes, except for sometimes showing a message that says the iPod's data is corrupted, that may be the problem.  I would put the iPod into Disk Mode

Putting iPod into Disk Mode - Apple Support

Then, with iTunes running, connect it to the computer (by itself).  If iTunes picks it up, it may prompt you to do a Restore with a message about "recovery."  If you're OK with erasing the iPod (all of the iPod's songs are in your iTunes library), tell iTunes to proceed with the Restore.  Hopefully, after the Restore, the iPod work properly again and you can sync your songs back.

I have exactly this problem - to a certain point.

I have been able to successfully transfer my iTunes account from an old HP Windows laptop to my iTune account, newly created on a new MacBook Pro.

I just want my existing iPod Classic 160 to sync with the new account on the new laptop.

When I connect the iPod to the new Mac it recognises the device, states 'Syncing', but then, after just a second or two, it states it is ejecting the device. It doesn't give me the chance or option to start a sync.

I have not had any messages to state that there is an issue with either my iPod or the laptop.

I feel as though I need to deauthorise my old laptop and authorise my new one, with respect to syncing. I have done this already with general library access.

I have tried the 'Disk Mode' advice but I get no recovery message thereafter.

I just want my new laptop to stop 'rejecting' my iPod and start the update to my existing iPod. Any ideas?!

You were using a USB hub when it did not work?  Some do not supply enough power to charge the iPod's battery and operate the iPod when syncing, and the hub can also become faulty.  Mine is OK, but if I connect more than two iPods (and other Apple devices) at the same time, I usually get error messages about iPods being "ejected."

If you buy a new hub, be sure to get one that has its own power supply (not the less expensive type that gets power from only the port).