When changing to 64bit mode make sure that the older Volume Group data is backed up as this was in 32bit mode and you will need to remove the Unit from the Extended tools for Remove contents of Unit (Keep in mind this will delete your data). Then set to 64bit mode and save settings from the Console tools > Boot options > sys. architecture and restart the system. Now you will be able to create a Logical Volume greater than 16TB or 20TB.