I don't get it. Your xbox was modded, so you didn't need to hotswap the original harddisk anymore.
QUOTE
Xboxhdm wont even recognise it and when I turn it on it goes to the multi language error 9 message screen.
Xboxhdm is a pc application, and the error 9 message is probably a xbox message. If it's locked, xboxhdm can't access it, unless you unlock it. If the xbox boots into an error 9, it won't unlock the harddisk.
Your bigger harddisk is also giving you an error 9? That has nothing to do with the harddisk being locked or not. Are you sure your harddisk flatcable isn't damaged?
If the bigger harddisk is recognised in your pc (being unlocked), it should give you an error 5 when you connect it to your xbox.
The original xbox harddisk is probably locked. You could try the dos atapwd program on it to see if it is electrical in order.
regards.