I've burned the Octopi bootable img 0.17.0 with balenaEtcher, but it doesn't contain a startup.nsh file, which I thought was needed by EFI Shell to boot.
Also, my experience with bootable drives in the past is that when I would mount them on my Mac, it would say it's unreadable and want to initialize it, instead this shows the file system on the drive.
Have the basics of a bootable drive changed that much in two years (when I last created a bootable disk)?
The OctoPi image contains two partitions. One is FAT32 and contains the boot files (mounted on the RPi as /boot). The other is a Linux partition which contains the rest of the filesystem.
The reason for two partitions is so that the files in the boot partition can be manipulated on Windows, MacOS, or Linux desktops prior to the micro SD card being booted on the RPi. The file most often manipulated is the octopi-wpa-supplicant.txt file. This file contains the SSID, password, etc. for the WiFi on the RPi.
With this partitioning, the RPi can remain headless (i.e. no keyboard, mouse, or monitor) from the beginning through to its normal life as the host of OctoPi and OctoPrint.
Thanks for the detailed description, that helps me understand what's going on.