On BackTrack wiki, it says "Format the USB drive to FAT32", but I don't know why, could that be the reason? Format your HD using FAT32. Also, check your security software, some have auto sandbox unknowing process, you may have turned off notifications. Unetbootin's official website have not mentioned anything about HD, only USBs. I have found alternative "Unetbootin", Universal USB Installer and LinuxLive USB Creator. Hopefully someone come with better seggestion.