You have 2 options.
1. You can install a modchip. Once that is done, you can backup the xbox eeprom and you can ftp to your harddisk the files needed to softmod the disk. You can remove the modchip once the softmod is operational.
2. You can build or buy a Eeprom reader. You can read your xbox Eeprom information with it. With this information and the xboxhdm package, you can install a softmod on your harddisk. You need the eeprom information because it contains the xbox unique hddkey which is used together with the hardisk model and serial number to calculate a unique hddpassword. That password is needed to unlock the xbox harddisk.
In case of hotswapping, you let the xbox unlock the harddisk after which you connect it to your pc. If you have the eeprom.bin file, the xboxhdm package can do the unlocking. You can safely connect the harddisk to your pc with the power switched off.http://forums.afterdawn.com/thread_view.cfm/357863