well hotswapping is always tricky, and not something i recommend relying on if you mod a lot of xboxes, but its good for emergencies
to hotswap to a hardmodded (chipped or TSOP'd) xbox put a slayers, AID or other rescue cd in the hardmodded box's dvd drive and turn it off, then unplug the HD
the unplug the unmodded box's DVD drive (unplugging the DVD drive forces an error 12, which leaves the HD unlocked longer than normal, making the hotswap easier) and turn it on, when it boots to the error screen unplug the IDE from the HD and plug it unto the modded box's IDE (leave the power plugged in to the unmodded box and leave it turned on)
turn on the modded box, and if all went well and the gods are smiling, it will boot the rescue cd and allow FTP access to the unmodded HD, so you can FTP the softmod
once the softmod is installed turn everything off, reassemble the components in their respective xboxes and and backup the eeprom from the newly-softmodded box for future use and/or building a new large HD