Your console's factory default BIOS won't let it boot unless the installed drive is locked. It's locked using a code derived from the drive's serial number and the unique code in your console's EEPROM.
Installing a modchip over rides the original BIOS, and the console will boot up regardless as to whether the drive is locked or not. You can manually lock it later if you wish (though there's no point unless you wish to use X-Box Live or disable the modchip).
The alternative solution (a bit more technical (or fun, depends how you look at it)) is to build an EEPROM reader
and manually prepare the new drive with the files it needs and that special lock.