A lot of people think Apple Silicon Macs can boot from external storage, and Apple themselves go to great lengths to pretend they can.
However, the iBoot bootloader does not have USB or Thunderbolt drivers at all, and absolutely cannot boot from external storage in any way, shape, or form.
But they're cheating.
When you "select" an external volume to "boot" from, whether from macOS or recoveryOS or the Boot Picker (which is just recoveryOS, which is just macOS), the fully booted OS with full access to external storage will copy the bootloader, firmware, and OS kernel to internal storage, then configure the machine to boot off of THAT. Then the bootloader is still just booting off of internal storage.
You can see this if you set up "external" boot, then try to power on the machine without the disk connected. The progress bar will appear below the Apple logo, and that progress bar is drawn by the macOS kernel, which proves macOS is already running, even though supposedly you removed the disk it's booting from. It only times out and fails a few seconds later when it can't find the external disk to mount the root filesystem from.
BTW, the only blocker for supporting the same exact mechanism for USB boot in Asahi Linux is that m1n1 does not have USB drivers either, which it needs to chain off stage 2 from USB. So if anyone wants to help out and write a bare-metal xHCI USB stack with enough support for hubs and mass storage devices in Rust... ;-)
@marcan could this could be a big problem if the internal SSD completely dies in that you can't then boot the machine at all as you can't then have the option of an external boot if the rest of the hardware is ok?
@smsm1 Hardware configs and boot policies are stored on the internal SSD, so there is no way for the machine to be functional without it.
@nicolas17 @smsm1 It's not, iBoot is in a discrete NOR flash just like on PCs.
However, a bunch of other stuff is in NVMe, such as system factory calibration, display calibration and configs, etc., never mind boot policies, so it still wouldn't work without it.
@marcan @nicolas17 @smsm1 has it been confirmed that it's actually separate hardware? Because a while ago someone was going around with the idea that it was somehow "fake NOR" and "mapped" onto an NVMe namespace. Sounded like bullshit, but...
@marcan @nicolas17 @smsm1 I think they called this "boot ≠ root" at WWDC back in the day.
TL;DR, It is indispensable to make backups of your macs, because data recovery is pretty hard when they die.
This is already the case since data uses hardware-tied encryption anyway.
However, given the inability to replace SSD, this imposes a cap on how long the machine may be used ?
You can replace the NAND module on Mac Studio models and on the newest M4 Mac Minis.
My MacBook Pro which serves as my main machine says 0% used (after a year+ of it being my main daily driver), so I'm really not worried. Apple SSDs tend to have significantly higher overprovisioning and reliability than the competition as far as I know.
@marcan i recommend you connect with the redox os team since they are also dealing with this