Install Big Sur onto EXTERNAL SSD drive

I have a 2017 21" iMac with rotating hard drive. Currently I am running Mojave. Up until a few weeks ago, when I read Glenn’s great article about how he rescued his similar iMac by using an external SSD for his boot drive, I hated my iMac. Now it is so responsive that I am considering updating to Big Sur. (Thanks Glenn!)

However, after reading a lot of related TidBITS items I am concerned about this plan.
Naturally I will want to NOT use the internal, rotating hard drive.
I will want to use the External SSD as the Boot drive and for it to have all the invisible partitions that Apple creates as part of moving beyond Mojave. I don’t want to use that mall-performing internal HD for anything!

Is there experience with using an external SSD drive for Big Sur? Is it good experience? Or should I plan on staying a Mojave for the future?

I have already resigned myself that I will no longer be able to create Bootable Clones once I move from Mojave

Thanks in advance for your information


I haven’t actually done this before, so others please feel free to correct/update me. From what I’ve read and from prior experience with older versions of MacOS, it should work, but you should be aware of a few things:

  • Make sure you have enough bandwidth for the SSD. At minimum, use a USB 3.0 device. Better, a USB 3.1-gen-2 (10 Gbit/s) device. Even better (but maybe too expensive) a Thunderbolt device.

    • If you are using USB, don’t share the port with any other devices. Let the SSD have all of the port’s bandwidth.
    • If you are building an external SSD from a SATA-based device, then Thunderbolt is overkill. SATA tops out at 6 Gbit/s, so just use a USB3 enclosure. If you are building an external SSD from an NVMe device, then Thunderbolt will give you much better performance, but it will cost more.
  • If your Mac has a T2 chip (which according to Apple does not apply to your 2017 iMac), then you must configure it to allow booting off of external media.

  • Since your Mac is Intel based, you can upgrade the internal hard drive and then make a bootable clone to the SSD. The latest versions of Carbon Copy Cloner should be able to do this.

  • You might, however, prefer to perform a clean installation to the SSD instead of an upgrade. (This is, I believe, your only option if you have an M1-based Mac). To do that, you should not just run the updater - that will upgrade your running installation (the internal hard drive). Instead, I have found what appears to be two different procedures. One involving an external bootable thumb drive, and one that does not.

    Both procedures start with formatting your external SSD and downloading the Big Sur installer:

    • Connect and format your external SSD. Use a GUID partition scheme and APFS format. (If you format it as “Mac OS Extended”, the Big Sur installer will convert it as a part of the installation process).
    • Download, but do not run the Big Sur installer via the App Store. Just download the installer app from the App Store.

    According to this MacWorld UK article, you should be able to tell the installer to use your external disk:

    • Double-click the installer
    • After agreeing to the software license agreement, it asks where you want to install it. Click on “show all disks” to see your external SSD. Select it. Then continue with the installation.
    • After installation, before rebooting, use the Startup Disk settings page to select your SSD as the new boot device.

    Alternatively, you may prefer to make a bootable USB thumb drive and use its installer. Especially if you think you may end up repeating the installation (perhaps to install onto multiple computers).

    • Create a bootable USB thumb drive from the installer.
    • Boot from the installer. Plug in the thumb drive, reboot while holding Option and select the installer as the boot device. It may require you to update your Recovery partition and reboot - let it, then remember to hold Option and re-select the thumb drive after the reboot.
    • Your Mac will boot into recovery mode. From there, you can use Disk Utility to format your external SSD, if you haven’t already done so (use GUID partition map and APFS format) and then run the Big Sur installer. It should let you select where you want to install it (click “show all disks” if your SSD is not visible) - select your SSD and continue with the installation.
    • When the Mac restarts (post-installation), hold down Option in order to select your new installation (on the SSD) in order to start from it. Then use the Startup Disk system setting to make that device the default.

    In either case, you will have a clean installation on your external SSD. Install/migrate your apps and data as you would if it was a new Mac.

Good luck. As I wrote, I have not actually done this before, so you might want to wait and see if anyone else replies with any corrections, in case I didn’t get it right.

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CarbonCopyCloner will let you make (sort of) bootable Big Sur clones…but you need to read the documentation on how to do it. They’re (at last time I checked) still waiting on Apple to fix something in one of the Apple tools to allow that although I haven’t looked in a month or two. Since the OS partition is separate and unwrite-able…just installing BS on the clone and then cloning just the non OS partition is essentially the same…the only drawback is that in order to update the OS on the clone drive you’ll need to boot from it and update the OS as I recall from the instructions at CCC.

What flavor of Thunderbolt does your 2017 have…you’ll want a TB drive that matches that and not a USB one. If it’s only TB2 then while it will be likely better than the internal spinning drive it might be as responsive as Glenn’s is.

Assuming you get the SSD hooked up…then you have two choices.

  1. Download the Big Sur installer and install on the external then use Migration Assistant to move everything over.
  2. Clone the internal to the SSD, boot from the SSD, and install Big Sur.

I’m considering a similar same upgrade myself on my 2019 iMac when/if the internal SSD runs out of space…along with considering replacing it with either the pro grade M-whatever mini and a monitor or the pro grade M-whatever iMac…likely the mini depending on size of the monitor on the pro grade iMac and the overall cost of each option.

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Thanks David. I knew I was going into slightly unchartered water

I will follow your guidance and wait quite a while for other responses before I try anything

I have a while as the only pressing thing is my Tax Software. The vendor says 2021 tax software won’t run in Mojave

It can make a bootable Big Sur clone on Intel Macs, but not on M1 Macs.

The only catch is that it can’t incrementally update the system volume, so after the first backup, you need to either make a clean backup of the system volume (copying everything) or only do the data volume.

But in this case, if the goal is to make the bootable volume and then use it as your only boot device, that’s just fine, since you won’t be making additional backups to it in the future.

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Re-reading my advice, it might be even easier to make a bootable clone of your existing Mojave system (which works just fine using Carbon Copy Cloner or Super Duper!) to the SSD, make the SSD your default boot device, and then run the Big Sur updater from there.

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Thanks again David

I already created the bootable SSD external Mojave clone and now run the 2017 iMac from the external drive as the startup disc

Your last sentence is the basis of my initial question. Will Big Sur go onto the external, startup drive without needing the damn, dumb, slow internal HD?
I’m just scared that the Big Sur installer ignores the current start up drive and does it’s thing(s) on the int HD

I did exactly this with my 2015 iMac. Bought a Samsung T5, cloned my internal Mojave fusion drive to it using SuperDuper, booted from the T5, and allowed Apple to install Big Sur on the T5. As far as I can tell it had no effect on the internal drive, and I can now boot from either one. I was careful to disconnect or turn off any backup drives/software so that there wouldn’t be any confusion about what drive was backing up to where. The T5 feels much quicker all around.

My goal was to see if any software I use was impacted by Big Sur. It all seems to work as expected but I’m still evaluating Big Sur so for day to day I’m using Mojave and have my normal 3 layered backup system operating on that.

There have been some issues with signing in to iCloud when switching boot drives and sometimes this caused problems with other synced Apple devices, but not a big deal. I’m basically waiting to see if SuperDuper/Apple will be able to do data backups from Big Sur before switching to that OS.

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Thanks I’m getting comfortable with the idea

Maybe it’s just your wording, but where’s the difference between “just running the installer” and downloading it first and then launching it? Isn’t the crucial point that you need to tell the installer it needs to install to another disk (and then select the external SSD) vs. just running it and have it default to installing on whatever you happened to be booted into at that time? Are you indicating if you run it straight from MAS it won’t give you the option to choose install location and instead directly install on the internal? That would be rather odd.


As I understand the articles I’ve read, when you download a macOS installer newer than what you’re running, the App Store (via Software Updates, I believe) starts the installation process immediately after download. If you proceed, it will automatically update the system you’re currently running and won’t give you an opportunity to select a different destination for the installation.

If you quit the installer instead (or don’t click on anything in the App Store or Software Updates to start the installation), then it will leave you with the installer sitting on your hard drive.

If you manually launch it from the Finder (or use it to make a bootable installer and then boot from that installer), then you are given an opportunity to select where everything should be installed.

The idea is that most people want to upgrade their running system, so this is made the default. You need to jump through a few hoops if you don’t want this default.

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Thanks again. I want it, eventually, on the external SSD
Now I have to digest ShirtPocket’s guidance about going back to earlier version of SuperDuper

This is correct. I do installation of MacOS Big Sur to test new software before the Graphic designers in the organisation I support get the updates. I download the installer and move it into the Application folder. When I launch it, it gives me the option to select the drive, I want to use. I use both internal and external USB drives. Here is a overview of different ways we sysadmins use to get this done. How to Download macOS Big Sur, Catalina & Mojave Full Installers I use the python script to download. The next time I do this, I will try this method:

Hi Jerry,

I just did exactly what you are asking about. I have a 2017 iMac 27 (18,3 is exact model fwiw) and am running Big Sur 11.3 from an external Thunderbolt SSD 2TB right now no problem.

Now my machine had no issues to begin with, and I already was running Big Sur, but I wanted the speed of a SSD without having to replace the internal drive.

I bought the OWC Envoy Express Thunderbolt enclosure at OWC ( $79.00 at, and added a Mushkin Pilot-E 2 TB NVMe ssd to it from (it was on sale for $215.00, but since sold out). I was trying to do this as inexpensively as possible, and still have decent performance. You can get whatever size NVMe stick you want in a pretty wide range of prices, or buy a “built” one from OWC and just plug and play!

In any case, I did a clean install of Big Sur from my recovery drive, and used Migration Assistant to pull over all my accounts and data. I’ve had no problems so far, It’s been 3 days since I made the switch. The Thunderbolt drive is the way to go I think, and my Mac boots really fast from it and apps launch in a blink of an eye!

I found it really pretty easy to do, good luck with it :+1:

Thanks for the news. I feel comfortable with doing the change in OS Just have to fix my back up procedures

I’m using that exact setup. 2017 27" iMac with internal fusion drive but now using external SSD as boot running Big Sur.

I gave a rundown of what I used and why in Glenn’s thread.

I do use CCC, but as someone else mentioned there was (is still?) some issue with Apple’s utility. I probably just did a fresh install onto the external anyway, as I like clean installs to clear out cruft.

That was probably me, referencing this article:

Carbon Copy Cloner: Yes, you can have bootable backups on macOS Big Sur

Apple’s utility can copy the system partition on Intel Macs, but not M1 Macs. It also can’t update the system partition, so if you need to update your backed up system partition you need to either blow it away and make a clean copy or boot from it and perform a software upgrade from there.

It looks like this will be fixed with 11.4; it is fixed with the 11.4 beta at least. See Dave Nanian’s recent blog post.


This was posted this morning:

TL;DR: macOS 11.3 solves some critical problems installing to external drives that existed in 10.2, but it’s still not 100% perfect.

I had done this about a year ago and installed Catalina. I upgraded to Big Sur at 11.3. Mainly was waiting because I use some open source libraries and wanted to make sure they were fine with Big Sur.

The following copied from About this Mac
Mac (Retina 5K, 27-inch, 2017)
External 1.92 TB Solid State PCI-Express Drive

OWC Envoy Pro EX 2TB Rugged, Portable Solid-State Drive with Thunderbolt 3. Not a cheap as @jsabini, but my notes say a 3-year warranty.

All that said, I might well buy a new 32-in. MX iMac if such a thing is released.

I already tried a slow SSD which I purchased to save photos. With Catalina installed it wasn’t all that different from the Fusion drive, but the fast SSD is much better.

Installation seemed just the same as for an internal drive. One thing that I have tried a couple of times is booting from the Fusion drive. A major pain because if you have any iCloud things opening you get endless password protection garbage (I know it’s trying to help, but this is the same machine in my home) and then you have to go through some of the same things when you boot back from the external drive.

The Fusion drive is of course there and the only downside is that the Fusion drive files show up on searches because it’s in the state it was whenever I started using the SSD. Maybe I should erase it. But it’s a free backup. It can’t be unmounted.