This is what I did:
manjaro-chroot -a
grub-probe: error: cannot find a GRUB drive for /dev/sdc1. Check your device.map.
grub-probe: error: cannot find a GRUB drive for /dev/sdc1. Check your device.map.
==> Mounting (ManjaroLinux) [/dev/sda4]
--> mount: [/mnt]
--> mount: [/mnt/boot/efi]
mount: /mnt/boot/efi: special device /dev/disk/by-uuid/04BA-87DD does not exist.
dmesg(1) may have more information after failed mount system call.
update-grub
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-6.5-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-6.5-x86_64.img
Found initrd fallback image: /boot/initramfs-6.5-x86_64-fallback.img
Found linux image: /boot/vmlinuz-6.1-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-6.1-x86_64.img
Found initrd fallback image: /boot/initramfs-6.1-x86_64-fallback.img
Found linux image: /boot/vmlinuz-5.15-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.15-x86_64.img
Found initrd fallback image: /boot/initramfs-5.15-x86_64-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
ERROR: mkdir /var/lock/dmraid
grub-probe: error: cannot find a GRUB drive for /dev/sdc1. Check your device.map.
grub-probe: error: cannot find a GRUB drive for /dev/sdc1. Check your device.map.
Adding boot menu entry for UEFI Firmware Settings ...
Root filesystem isn't btrfs
If you think an error has occurred, please file a bug report at "https://github.com/Antynea/grub-btrfs"
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
I checked the /etc/default/grub file is as you said and it is