This is probably IIRC a Calamares limitation.
- Try the installation with manual partitioning.
- Select a vfat/fat32 partition on external disk (flag as boot).
- If possible (in Calamares options), don't install bootloader. This may not be an option, or may fail again anyway.
- After the end of installation (even with the same error), don't reboot.
- Exit Calamares and use File manager to check there is a system installed on external drive.
- If so, chroot into the (external drive) system with garuda-chroot and install grub manually, by Archwiki instructions (or reading grub manual)
garuda-chroot is not installed, (or anyway) install it without system upgrade.
sudo pacman -S garuda-chroot
About installations on external drives, you cannot (practically) use nvidia proprietary drivers, just using open-source nouveau driver.
Either don't install any video driver, so the system will use kernel embedded (open-source) modesetting drivers, or install
mhwd utility, which will install the same drivers' packages.
Kernel default (first/main grub entry) images include HW drivers for HW that is present at the time of image building (mkinitcpio).
If you plug this (external drive) system on a different PC, with different accessories (mouse, keyboard etc) and some HW is not recognized/used, try booting to the fallback grub menu entry, which includes a richer drivers collection.