grub not working after EndeavourOS installation (via live USB)

Question

I installed EndeavourOS (based on arch linux) via live USB following the guide on my /dev/sda partition where I used to have Ubuntu, which worked fine. I also have a separate HDD, my /dev/sdb partition, with Windows 10 installed. Everytime I turn on my PC I get to the grub shell and I can't boot normally into neither of the OS.

This is the output from grub shell:

grub>ls
grub>(hd0) (hd1) ... (hd1,gpt2) (hd2,gpt2) (hd1,msdos1)...

The partition I need is (hd1,gpt2) so I do:

grub>set root (hd1,gpt2)
grub>linux /boot/vmlinuz-linux root=/dev/sda2
grub>initrd /boot/initramfs_linux.image
grub> boot

and it boots normally into EndeavourOS. Then I try to fix my grub installation:

$ sudo grub-mkconfig -o /boot/grub/grub.cfg
$ sudo grub-install /dev/sda

And I then get this error:

Installing for x86_64-efi platform.
grub-install: error: cannot find EFI directory.

I have tried specified the EFI partition following this answer:

$ sudo mount /dev/sda1 /mnt
$ sudo grub-install --efi-directory=/mnt/EFI

but I get:

Installing for x86_64-efi platform.
grub-install: warning: disk does not exist, so falling back to partition device /dev/sda1.
grub-install: warning: disk does not exist, so falling back to partition device /dev/sda1.
grub-install: warning: disk does not exist, so falling back to partition device /dev/sda1.
grub-install: error: disk `hostdisk//dev/sda1' not found.

I checked the /mnt/EFI directory:

$ls /mnt/EFI/
drwxr-xr-x 2 root root 4096 21 apr  2019 BOOT
drwxr-xr-x 3 root root 4096 24 mar 15.22 ubuntu

So I thought maybe the usb live installation didn't format correctly the partions... Is there a way to fix this rather then format everything and start from scratch? Thanks in advance.

EDIT: I tried as suggested to refer to /mnt as the efi directory:

$ sudo grub-install --efi-directory=/mnt

and got this error:

Installing for x86_64-efi platform.
Could not prepare Boot variable: No space left on device
grub-install: error: efibootmgr failed to register the boot entry: Input/output error.

Now my /mnt/EFI folder looks like this:

$ ls /mnt/EFI
BOOT  endeavouros  ubuntu
$ ls /mnt/EFI/endeavouros
grubx64.efi
$ ls /mnt/EFI/ubuntu
BOOTX64.CSV  fw  grub.cfg  grubx64.efi  mmx64.efi  shimx64.efi

should I maybe delete the ubuntu folder?

also I have a ton of results running efibootmgr:

BootCurrent: 0000
Timeout: 1 seconds
BootOrder: 0000,000C,001A,0012,0007,0009,0011,0013,0014,0015,0017,0018,0019,0001,0002,0003
Boot0000* ubuntu
Boot0001* UEFI:CD/DVD Drive
Boot0002* UEFI:Removable Device
Boot0003* UEFI:Network Device
Boot0007  UEFI OS
Boot0009  CD/DVD Drive 
Boot000C* UEFI OS
Boot0011  UEFI OS
Boot0012* Unknown Device
Boot0013  ubuntu
Boot0014  ubuntu
Boot0015  UEFI OS
Boot0017  ubuntu
Boot0018  UEFI OS
Boot0019  ubuntu
Boot001A* UEFI OS

SOLVED: I followed the guide on the wiki, launching the live system in UEFI (otherwise it won't work).

Answer 1

The efi directory is wrong and should be set to the path where the partition is mounted. Also use sudo for grub-install:

sudo mount /dev/sda1 /mnt
sudo grub-install --efi-directory=/mnt

Edit:

It looks like your efivars (the EEPROM location, not the partition /dev/sda1) is full and you need to clean up.

Run efibootmgr -v and remove obsolete entries, i.e. if there is an old Ubuntu entry like

Boot0014* ubuntu    HD(1,GPT,...)/File(\EFI\ubuntu\shimx64.efi)

then you may remove it with

sudo efibootmgr -b 0014 -B

Also delete dump-* files in /sys/firmware/efi/efivars, see this answer: Grub installation failed.

Yes, you may also remove the /mnt/EFI/ubuntu directory, but I don't think it's the cause of the error since endeavouros/grubx64.efi was successfully written.

Then try

sudo grub-install --efi-directory=/mnt

again.