Hi,
error 6 means that you succesfully locked it, but not with the right password.
So, unlock it first. Xboxhdm also sets a Master password which you could use to unlock the harddisk.
Make sure you have the correct Eeprom.bin file for your xbox. You can save it on a floppy or you can put it in a folder before you create the Iso and burn the xboxhdm boot CD.
Some say it should be in the Eeprom folder, others say it should be in the E:\backup folder.
regards.