While I’m sure there are a multitude os answers to your question, the two which come to mind are:
1. Check you motherboard BIOS settings – the boot order should be able to be adjusted to allow ONLY boot up from the new HDD (dev0, most likely).
2. There is a debug routine which will overwrite the boot sector of a hard drive and relocate it (useful if the hard drive is physically corrupt or if there is a boot sector virus). The script can be modified to simply erase the boot sector.
There are probably a dozen more ways to do this – let me know if you want the script…