Cannot restore after hdd replacement - not detected by computer

I bought a used ipod off ebay so I could replace the battery and hdd

having done the replacements I can't get the ipod to work again

The ipod powers on and either displays a sad ipod icon or tells me to connect to a pc to restore on itunes

When I connect it up to restore on itunes it powers on and displayed the do not disconnect screen for a while before rebooting to show the sad ipod icon

The device is also not being detected by itunes or showing up on any computer I connect it to.

I'm starting to think that this is a logicbord or hd cable issue. Is there a way to tell which it might be or is there a step I have missed