Q: itunes could not copy song to ipod because unknown error occurred (-5000)

See Re: Corrupt iPod classic.

tt2

It's not stated what you have already tried, so...  Shut down (power off) computer and disconnect all USB devices.  You can leave standard USB keyboard/mouse connected, if used.  If your computer is a laptop, connect its power adapter so that it's not running on battery power.  Do this to Reset (restart) your iPod

Learn how to reset your iPod - Apple Support  (see section iPod with a click wheel)

Start up computer, run iTunes, and connect iPod to a direct USB port on computer, by itself.  Then try to sync it again.

If that does not help, do a Restore on the iPod using iTunes.  This erases the iPod, reinstalls its software, and sets it to default settings.

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

Then, connect iPod to a direct USB port on computer, still by itself, and try syncing again.

Have you tried to enable disc use.

Maybe this information helps you

If you see error 1415, 1417, 1418, 1428, 1429, 1430, 1436, or 1439 when restoring or updating an iPod in iTunes - Apple …

See Repair security permissions for iTunes for Windows.

Check the hard drive with the source media for errors.

See the break-up large transfers method in Re: Corrupt iPod classic.

tt2

One possibility is that your iPod's hard drive is becoming unreliable.  That is why the iPod does Restore fine and initial syncing works.  But part of the hard drive's storage media is faulty, and when a large transfer (like a full library sync) gets to that certain point on the hard drive's storage media, errors occur. 

uninstalled and reinstalled itunes

If you did this already, you probably did not completely remove iTunes and related software components.  This Apple document lists the components of an iTunes installation, and the precise order to uninstall them.

Remove and reinstall iTunes and related software components for Windows 7 and later - Apple Support

After reinstalling the latest version of iTunes from here

https://www.apple.com/itunes/

try doing a Restore and syncing again. 

If you still have a problem, here's a test that takes iTunes syncing out "out of the picture."  Do another Restore.  In iTunes, on the iPod's Summary settings screen, Enable disk use (and Apply) if it's not already.  The iPod's "disk" appears in Windows as a USB storage device.  Quit iTunes.  Use Windows to copy a large amount of data to the iPod.  You can create a folder on the iPod called Test and copy large files (or folders with large files) to that folder.  It doesn't matter what data you use; you can even use your iTunes Media folder, or maybe a folder that has video files.  Keep going until you fill up the iPod.  At some point, do you encounter similar problems with copying the data to the iPod, or does it work smoothly without stalling or errors?

What are the HDD stats?

tt2

No reallocations or pendings shows the iPod's drive is healthy. Perhaps it is the source drive that has a problem?

tt2