Elementary os fatal error extracting grub

Hi , is all day that im trying to install elementary os on a external disk connected to usb .
i have 1 usb memory of 14 gb with elementaryos-5.0-stable.20181016.iso downloaded from original site.
i tried to mount the iso with MBR and GPT partitons same result
so i checked my bios like no fast boot , and no secure bot.
starting the PC pressing F11 in my case , selecting the usb with the iso.
loading elemetary live
select install ok
select to install in other ..
creating the partition for external disk formatted in NFTS
frist attemp:
primary , beginning ext4 with the root / with slash
primary , beginning ext4 with the root /home
select to install on the external disk
got critical error when extrating the grub 2 during the installation
also tryed to install the swap memory like this
second attemp:
logical , end swap partiton
primary , beginning ext4 with the root / with slash
select to install on the external disk
got critical error when extrating the grub 2 during the installation
please i need a really easy to understand guide step by step