Tuesday, December 22, 2020

Booting an M1 Mac From an External Disk

Howard Oakley (tweet):

The most obvious method of doing this is to run the Big Sur installer app, and select a freshly formatted external drive as its destination. Some who have attempted this using an M1 have reported that the installation never completes; thankfully mine was completely successful until I tried to restart from it. Although I could see and select it in the Startup Disk pane, Big Sur 11.0.1 couldn’t be booted from there.

Checking the external disk gave no clues as to what was wrong.


zero problems[…] But [don’t] support X86 Mac installed partition, you need full new install on M1 Mac.

Howard Oakley (tweet):

Thanks to many comments but no real clues, I’ve now been able to solve this, and here explain how you can do this too.


Once booted from the internal disk again, ejecting the bootable external disk isn’t fault-free. Clicking on the Eject tool for the System disk brings a helpful dialog which offers to unmount both System and Data volumes. Oddly, on Intel Macs running Big Sur, the Data volume isn’t shown as a separate volume, but on M1 Macs it is.

Unfortunately, on an M1 Mac running macOS 11.1, the Data volume is usually left behind, and any attempt to eject it will result in your being told that it’s still in use, while trying to open the volume denies that it’s present at all.


If you’re trying to create and use a bootable external disk for M1 Macs, I therefore recommend that you don’t waste your time discovering which USB-C SSDs might be compatible: go straight for a Thunderbolt 3 enclosure. It’ll cost you more but save you time and effort in the long run.

Update (2021-01-01): See also: Hacker News.

Update (2021-02-10): Howard Oakley (Hacker News):

Now that Big Sur can be updated to 11.2, I’ve been looking at how that affects external boot disks, and whether it finally enables them all to work. However, far from 11.2 solving this problem, it actually makes it worse.


It doesn’t appear possible to update a bootable external disk, only to format it and install a fresh system, as if the disk had never had macOS installed before.

The 11.2 full installer is incapable of copying existing account settings from the internal SSD to a fresh installation of 11.2.

M1 Macs currently don’t work reliably with bootable external disks. Users who want to boot from an external disk would be wise not to buy an M1 model, until Apple has fixed these problems completely.

Update (2021-03-14): Howard Oakley:

Having now completed the most pointless macOS update ever – that to 11.2.2 – I had hoped that Apple might at least have finally fixed the bug which prevents macOS updates from being installed successfully on most external bootable disks connected to M1 models. TL;DR: it doesn’t.

Update (2021-03-23): Howard Oakley:

If you want to use 1 True Recovery to change to or from using a bootable external disk, use its Startup Manager. Press and hold the Power button until the display shows Loading Startup Options, then release it. This takes you to the Startup Options screen. Wait until all bootable disks have loaded into the list, select the disk you want to boot from, then click Continue underneath it.

Howard Oakley:

If there’s anyone left who’s still trying to boot their M1 Mac from an external SSD, the question now is how to update it to macOS 11.2.1. As I reported, booting from the external disk isn’t likely to result in successful update, although a few users have reported that Software Update worked first time for them. Assuming that you have the same bad luck that I’ve had trying to do this on numerous occasions, here are my findings to guide you.


Update (2021-04-15): Howard Oakley:

With changing Mac hardware, the use and value of external boot disks is changing. This article is a short overview of what’s happening, and its consequences.


Update (2021-05-03): Howard Oakley:

Big Sur 11.3 progresses support of external bootable disks by M1 Macs, but can still appear flawed and unreliable, at least with non-Thunderbolt SSDs.


Strangely, with both bootable external SSDs (Thunderbolt 3 and USB-C), changing between booting from the external or internal SSD often takes you through the Recovery Assistant, despite the Mac being restarted from the Startup Disk pane. Although not a problem, it means that switching between disks can require two restarts.


The Finder often fails to eject external volume groups correctly. Although the ejection itself works, the Data volume is usually left in the Finder’s sidebar, from which it can’t be removed until the Mac is restarted without that external disk connected.

9 Comments RSS · Twitter

One of the reasons I stuck with the Mac for several years was the ease of bootable external backups, so to see this very convenient feature become arcane and complicated is a total letdown. Just like with text rendering, PDF rendering, stability, and general UX, Apple seem dedicated to removing every advantage that macOS previously had.

Meanwhile, I can make bootable backups on Windows with the same cheap USB 3 drive toaster I've been using with my Mac. And I'm still using it with macOS, just on Mojave, which also supports bootable HFS+ backups on affordable HDDs.

Stepping back and thinking about it, it's amazing just how much money Apple have implicitly asked users to spend, just to work around their new limitations, bugs, and lack of quality. SSDs instead of HDDs for backups, Thunderbolt enclosures instead of USB 3, high-DPI displays instead of standard DPI displays... we're looking at potentially thousands of dollars of unnecessary purchases, just to keep using our existing computers without hassle.

Apple wants to force all your data into iCloud, regardless of how badly that shafts customers in terms of privacy and budget. Soon: no more backups larger than [n] GB without a paid subscription.

I do suspect Apple would like to have iCloud as a Mac backup option in the long run, but with a maximum capacity of 4 TB (for an entire family), that's clearly not practical (for everyone) any time soon when. They don't seem to be in a hurry.

I don't think this is about that at all — it's just their usually imbalanced view of security vs. usability. The days of drag&drop of a folder literally named 'System' to a different volume are, sadly, long gone.

I can leave my own anecdote concerning this. I successfully installed 11.0.1 on an external drive without much effort at all, and with only one hiccup. It was on a 500 GB SSD that has four partitions: one for Windows, one for Catalina, one for data, and one for Big Sur.

I formatted the Big Sur partition as APFS using Disk Utility, booted into the Big Sur recovery environment, and then installed Big Sur on the fresh APFS partition. The first time the installer froze partway through for no reason. The second time it worked fine. Given Apple's track record these days, they cleared the already low bar by it only taking two attempts.

It's definitely true that installing and managing macOS system partitions has become way more buggy, more of a hassle, and less consistent since Catalina, and even more so with Big Sur. It's really too bad Apple is jettisoning so much of what made them better than the rest. Installing Windows on an external drive is kind of a nightmare and no longer supported by Microsoft. And booting Linux partitions in anything other than a simple configuration is annoying if you're not already intimately familiar with grub. macOS used to make it dead simple, but not so much any longer.

It's not just the iCloud size limit that's problematic. Every aspect of backup-ability is suffering today.

Time Capsule was discontinued in 2018. Time Machine couldn't use the current Mac filesystem for the first 3+ years. As of last month, you can now use APFS, but there are some complexities you have to understand in order to use it. It's not as simple as "any Mac-formatted hard disk you have sitting around will work", like before. Third-party backup solutions have been running into design, implementation, and documentation issues with APFS and recent versions of macOS, too.

It's quite obvious that backups, in any form, are a much lower priority to Apple than pretty new icons.

It’s astounding watching MacOS get less flexible over time. M1 has people excited about the performance of Macs again, but once the newness of that wears off, I suspect we’ll all be back to complaining about the UX and stability regressions.

I tried both macOS 11.1 & 11.2.beta on 2017 iMac 18,3 & 2013 MBP 11,1
Booting off USB stick setup by createinstallmedia worked fine and the installation onto external USB drive proceeded until got stuck at "Less than a minute remaining" though the activity light on the external USB drive kept flashing.

NB on iMac booted off USB stick, neither Magic Mouse 2 nor Mathias wired keyboard worked. Used 3rd party USB wireless mouse.

Tried slightly different route on MBP.
Booted off USB stick and went into terminal
sudo ditto --nocache /Volumes/Macintosh\ HD/ /Volumes/
makes full backup (I think)
Then used Disk Utility to erase /Volumes/Macintosh\ HD/
and installation of macOS 11.1 onto internal flash drive went smoothly.

Thankfully my MBP was not bricked.

Please Apple, cut back on annual release cycle and fix bugs. I'm sure you've see comments like
* https://rachelbythebay.com/w/2020/12/22/thunderblah/
* https://macperformanceguide.com/autoTopic.html?dglyKW=Apple+Core+Rot
* https://mrmacintosh.com/macos-big-sur-11-list-of-serious-install-upgrade-issues-updated/

I found "Use macOS Recovery on a Mac with Apple silicon" explains most of the install stuff for a M1 Mac (7 pages A4 print). The USB stick install is gone. Read the doc and careful with the ssd wipe (did it twice before I had the vol/containers right). DON'T touch the recovery volume :-)


Nick Quinn

Thanks for keeping this post updated with the Saga. I just bought my first PC in over 20 years and found it was surprisingly easy to boot from external media.

The ease of booting from external drives, and even booting one Mac off another in target disk mode, used to be a major advantage of Macs for me, but now the tables have turned.

Leave a Comment