2025-04-16
a quick PSA to those eyeing up the storage prices on the new M4 / M4 Pro Mac minis: did you know you could do this?
in my case I grabbed these two (not sponsored):
and a copy of macOS off the Mac App Store.
one “Install macOS” later, and we have a copy of macOS on an external drive. cool!
or one would think. see, I made the mistake of setting up my external drive on a different Mac.
on an Apple Silicon Mac, your external drive needs to be “blessed” for it to boot. when the M4 Pro Mini arrived, I attempted to use Startup Disk to set it as my primary boot device, hopefully “blessing” it in the process.
⚠️ ⚙️
Unable to set startup disk
An error occurred while setting “meowbase one” as the startup disk. The operation couldn’t be completed. (SDErrorDomain error 104.)
cool.
no, it turns out, the solution was to go into Disk Utility, create another APFS volume- “meowbase two”, Install macOS again, and use Migration Assistant to yank the data over from the old to the new.
finally!
…but not before my fresh out-of-box Mac Mini turned itself into a brick.
see, this time I was using Recovery (holding down the power button) to get to Install macOS. foolishly, once the install completed, I fired up Disk Utility, still in Recovery, and went to erase the internal SSD. that was the wrong choice.
it did as it was told, and somehow, bricked the internal SSD in the process.
the boot process of an M-series Mac is heavily linked to its internal SSD, as I alluded to with the whole “blessing” bit. see, it doesn’t boot straight off the external disk based on some flag in NVRAM. no, what it does is boot into Recovery, Recovery loads all necessary drivers, GPU, Thunderbolt, NVMe, what have you. shows you a nice progress bar with an Apple logo. Recovery then performs a bunch of security signature checks before chainloading into your external disk and handing off control.
only problem: my Recovery was gone. and my shiny new M4 Mini was blinking orange SOS at me.
okay, fine, not great. but I have a second Mac to recover this one. plugged it in as per Apple’s support documentation, using the chunkiest cable I owned (another Thunderbolt 4 cable). sure enough, it popped up as DFU in Apple Configurator.
i clicked Restore and waited for it to download/upload its 15 GB restore image, which i’m pretty sure it was doing at USB 2.0 speeds. it eventually finished, and…
⚠️
The System cannot be restored onto this device.
The operation couldn’t be completed. (com.apple.BuildInfo.preflight.error error 21.)
crap.
some Googling later, I found myself on the Apple wiki.
the following stood out to me:
You have to put your device into the DFU mode and not into recovery mode.
well, it claimed it was already in DFU mode. having watched enough dosdude1 NAND swaps, i remembered something about the board config / serials being stored in NAND. maybe those corrupted a self-initiated DFU mode?
once again following Apple support, I unplugged power from the Mini, then plugged it back in holding the silly power button on the bottom. sure enough, it showed up as DFU in Apple Configurator again, and this time the Restore finally went through.
I haven’t tried to erase the internal SSD again, to avoid tempting fate. instead I simply use a couple fstab lines to prevent it from auto-mounting on boot, and that’s that:
# Prevent auto-mount:
# Macintosh HD (System)
UUID=2B0B2D97-BF8D-45E7-A07F-EC3FC7330ADC none apfs rw,noauto
# Macintosh HD (Data)
UUID=2758F2A5-C76E-4C4B-9096-D3680BCD3C5E none apfs rw,noauto
don’t let this discourage you from installing macOS on an external SSD. once it works, it works. I haven’t had a problem with it since. and it’s really nice not to think about where Photos or iCloud or whatever is storing its things. I get to use my “internal”2 disk as a scratch drive without thinking about it while doing Blu-ray encodes. it definitely beats out the alternatives of:
no, I now just have 4 TB of “internal” SSD storage and I saved myself a kilobuck in the process. which of course I used to spec out all other parts of the machine.3
actually Thunderbolt 3, for the purposes of data transfer they’re the same spec. ↩︎
about the Thunderbolt 3/4 speed hit: I can see it in benches but I can’t feel it. but this thing also supports Thunderbolt 5 when those enclosures come down in price. ↩︎
I got bit by 16 GB of RAM in my M1 Pro. despite what everyone else says, macOS’s OOM handling is abysmal. it happily murders my FiiO USB DAC’s audio driver every time it OOMs. ↩︎