It doesn’t need to be F2FS. When you don’t specify a filesystem format, mount tries the filesystems that are available and F2FS is particularly verbose.

Try passing the filesystem type that you want to mount by using -t option.

You can get the list of available filesystem drivers by reading /proc/filesystems. If you don’t see the one that you want, you’ll need to update the Linux kernel configuration to included it. I’m pretty sure that vfat is what you want and that should already be enabled.

Also, as you noticed erlinit runs way too early in the boot process and Linux hasn’t enumerated the USB drive yet. Mounting filesystems in erlinit is really only intended for pseudo filesystems (like those under /sys) and things needed before any Elixir code is run (like the application partition so that the the command line history file can be initialized or read).

For posterity: after lots of playing around, I realized that I had created the USB sticks with a gpt partition table. After switching the disk to trusty MBR + exFAT code (07 in fdisk), and creating the disk using mkfs.vfat mounting worked. IMO the base images should include support for GPT partition tables though!

Finally, I was able to mount the hard drive. There’s an issue related to version or architecture differences between x86_64 and arm64. I cross-compiled the e2fsprogs, copied the mkfs.ext4 to the nerves image, formatted the hard drive and I was about to mount it without any issue.