I think you need the eeprom chip to unlock the HDD. A modchip may be able to unlock it some how, but if it doesn't you can always use the eeprom chip from the same xbox that you got the HDD out of.
You will have to unsolder the eeprom chip from the xbox (the one you got the HDD from) and then solder it into the other box (one you plan to use the HDD in).
If you download the xbox repair guide V2, it has a guide on how to remove the chip and install it. It also has a video on how to do it.
you can always buy a new HDD that is unlocked and then use it with the mod chip, but I think you need the eeprom xhip to unlock the old xbox HDD.
I could be wrong about this though, but I think you need the eeprom chip to unlock the HDD.
