Syncing issues after restore

Unfortunately, your earlier symptoms (songs skipping) and your new ones may point to the iPod having a faulty hard drive. Doing a Restore was the correct action, because the cause for the skipping songs may have been data corruption. After the Restore, the problem with not being able to complete a sync may be due to a problem with the hard drive (iTunes is not able to properly write data to it), which was also the cause for the data corruption (the iPod was not able to access its stored data).

You can try doing an Erase using Disk Utility, to completely reformat the iPod's hard drive. Select the iPod in iTunes and set it to Enable disk use (on the iPod's Summary settings screen), if it is not already. Quit iTunes. Run Disk Utility. In the Disk Utility sidebar, select the iPod DEVICE, NOT the volume indented below the device. Click Erase. For Scheme, select Apple Partition Map (NOT the default GUID Partition Map). For Format, select OS X Extended (Journaled). For Name, it does not matter. Click Erase.

After the Erase completes, quit Disk Utility and run iTunes. The iPod does not have its on board software, so you need to do another Restore using iTunes. After the Restore completes, set it up and try syncing. If you still have the same problem, the most likely cause is a faulty hard drive.

If your iPod's hard drive is faulty, and you want to repair it, those hard drive components are no longer being produced. Instead, you can consider using commonly available (and recently quite affordable) flash-based storage in the form of compact flash card and SD card, on an adapter. This web site has interesting and relevant information (including some demo videos).

https://www.iflash.xyz

I have not used these particular adapters with 5th gen and later "classic" iPods, but I have performed similar mods on my even older 3rd gen iPod, 4th gen iPod, and iPod mini (with different adapters). They work fine with flash storage replacing the mini hard drive, and actually work better now, with increased storage capacity. The iPod mini is especially impressive, going from only 4GB to 64GB.

The hardest part, for 5th gen and later iPods, is opening the case. Use the proper tools to avoid ugly damage. OWC's listing for a replacement battery has a HOW TO video showing a good technique

http://eshop.macsales.com/item/NewerTech/BIPOD4505C/

This web site has illustrated repair guides

https://www.ifixit.com/Device/iPod_Original

Doing an Erase using Disk Utility completely erases the iPod's hard drive, like it's a USB external drive or flash drive. The iPod does have some code in its firmware (on the logic board) for the most basic functions (like what it does immediately after you Reset it). But the software for how the iPod operates (as an iPod) is stored on the iPod's hard drive.

If you do an Erase using Disk Utility, you need to run iTunes and do a regular Restore on it, to reinstall its onboard software on its hardware, Otherwise, the iPod is just a storage device (like a USB external drive or flash drive).

After you do the Restore, if you expect it to sync your songs automatically, you need to set it up to Sync Music (the default setting is to use manual loading). Select the iPod in iTunes (click its device button on horizontal bar) to show its settings screen in iTunes window. Along the left side (in the sidebar), under Settings, click Music. To the right, the iPod's Music settings screen is shown, where you can turn ON and set up automatic syncing.

The thing that replaces the stock mini hard drive is the combination of the adapter and the flash card. You can use either a compact flash card or SD card (or even an m-SATA card) - different adapter for each. The adapter is just an adapter; it converts the iPod's internal hard drive connection to allow the flash card to be used for storing its data. That iFlash web site has some demo videos and additional details

https://www.iflash.xyz/iflash-dual-uploaded-some-videos/

https://www.iflash.xyz/ipod-and-sdhc-sdxc-cards/

SD card seems to offer (typically) lowest cost for the same capacity, compared to compact flash card (and m-SATA card); I recently bought this one at 128GB, for use with my Mac mini (not an iPod) for less than $50

http://www.amazon.com/PNY-Elite-Performance-Speed-P-SDX128U395-GE/dp/B00WWBCQEI

My iPod mini now uses the 64GB SD card (on a different adapter) that I previously used with my Mac mini. It seems like they (SD cards) are also more power-efficient (from my experience with my older iPods), which is important for use in a battery-powered iPod. You don't need to buy the one with the fastest specs. In this case faster is not necessarily better, because the faster one may require more power. And the iPod's stock hard drive is quite slow.

Make sure you do it as I described in the first reply, with the iPod device selected in the Disk Utility sidebar (not the indented volume). It possible the iPod classic holds more code in its firmware, enough to show its screen menus. My personal experience is with older hard drive iPods.

If you did that using Disk Utility, your iPod's hard drive is erased. Do a Restore in iTunes after that, and try to sync it and use it normally.

Here's a test you can do, to trouble-shoot the problem with the delay when opening iTunes, to narrow down and isolate possible causes.

Create a new OS X user account in System Preferences Users & Groups pane. The "Test User" account can be a Standard (or Administrator). Log out and log in to the new user account. Do not use Fast User Switching.

Set up the new user account like your primary account, linking it to your Apple ID. Run iTunes, and sign in using your Apple ID. This is a new iTunes library at this point. If you have songs purchased from the iTunes Store, you can use the Purchased screen to add some of your past purchases to your iTunes library

Download your past purchases - Apple Support

When you quit and relaunch, do you experience any delays? If you do not, the cause of the problem is something in your usual account, such as your iTunes library database or third-party background processes that run when you log in. If the same delay does occur in the new user account, then the cause is at the system level (or maybe something related to your Apple ID in this case); that would be the OS X system, or the overall iTunes application.

One thing to check for with iTunes (at the system level) is "plug-ins." Look here (using Finder)

[startup disk]/Library/iTunes/iTunes Plug-ins/

for any iTunes "extensions" from third-party developers (not Apple). I have only one item there; it is called Quartz Composer Visualizer.bundle. Doing a Get Info on it shows it's from Apple. If you have anything there that is not from Apple, and it's something you don't need, remove it. See if the delay goes away.

Another thing you can try (at the system level) is to reinstall iTunes. Go to the iTunes download page

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

NOTE: If you are intentionally using an older version of iTunes, this gives you the latest version.

Download the installer and run it. You should always have a backup of your iTunes library data (along with the rest of your user data), but reinstalling the iTunes application does not affect your iTunes library data. (Start using the built-in Time Machine feature, if you are not already, for automated backups of your entire system.)

If the new user account test points to problem at the user account level (not system level), one place to check is your Login Items. This is at System Preferences Users & Groups pane Login Items tab. These are processes that start up automatically when you log in to your account. One of the items is probably iTunes Helper, and that's fine. If you see any other item(s), and you don't know what it is or what it does (or it's for something old that you don't use anymore), you may want to remove it (them) from the list (select it and click minus sign). Don't remove something that you are currently using. If you made a change, log out and log in (or restart) and see if there's any difference with iTunes.