ZLD1016

My friend gave me his old, broken 20 gb photo ipod, ( i think its the 4th gen?), i thought i would try to fix it for fun. it showed a battery symbol so im guessing it was a battery problem. so i took apart the ipod and took out the battery and the hard drive and then put them back it and put the ipod back together. when i turned it on it said it needed to be charged so i charged it and it started working again, but had no music on it. i restored the ipod back to factory settings through itunes and tried to put music on it. the ipod froze so i unplugged it. now every time i plug it in my itunes freezes and when i unplug the ipod my itunes works again and a message comes up telling me to restore the ipod again. i tried setting the ipod to disc use (restarting it then pressing play and the middle button) when i do that i can put files on the ipod (i put a song in the notes folder and when i unplugged the ipod it was there but i couldnt play it) i have no idea what is wrong with the ipod, it holds a charge and everything seems fine until i plug it in. please help!!!