iPod 4th/5th Generation can't be customized

Hi there,

A few years ago I purchased an iPod Nano, I'm not sure whether it is 4th or 5th generation. I customized it on Windows and lost it when I moved. I just found it today and wanted to customize it on my new MacBook Pro (old Windows computer is dead long ago).

But iTunes won't let me delete selected photos on my iPod, and when I wanted to sync over new music, it informed me that all my other music will be deleted!

Also, on my Apple Account, it somehow doesn't show on my list of registered products. I KNOW I registered it when I bought it. I tried to add it to the list, and it wanted to know the exact date I bought it. I don't remember, and it won't let me add my iPod to my list of products.

What can I do about all this? I do not want to lose precious data in the process of deleting the ones I don't need. And I want Apple to know this product is mine, yet it won't let me add it to the list.

Can someone help me please?

Thank you!