Install Big Sur onto EXTERNAL SSD drive

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.


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.

I had the same issue with searches. Go to Spotlight (in System Preferences) and make the other drives not indexed

What external SSD would be recommended to run Big Sur on an external? Is USB enough or will I see a big difference if going with a Thunderbolt connection?
My machine is a 27" iMac 2017.


I used the 1 TB SSD that Glen mentioned in his article that I referenced. He had mentioned an OWC Envoy Pro EX that listed for just under $300.00. That is enough for me as my internal, slow hard drive is 1 TB. I’ve heard from some that they did a 2 TB SSD

I would NOT do this with USB drives. In fact the OWC SSD is only with a Thunderbolt 3 connection.

It is amazing how improved my iMac is

@jsabini I’ve got the same iMac as you and have been thinking of doing exactly what you did. Once you moved the accounts and data from your fusion drive, did you erase it or keep MacOS on it? If the latter, do you try to keep it in sync with the SSD? And are you doing nightly replication? I’m currently running Catalina on the Fusion drive and use SuperDuper! for nightly replication to an external USB 3 drive. Thanks!

Hey Andrew,

I did not erase my internal drive, I might down the road, or more likely use it as a data backup destination for the data partition, I’m not sure yet. I’m a little hesitant to nuke the internal drive, as I like having the peace of mind knowing I have another bootable drive I can count on.

I already had 2 external USB 3 drives as backups, one using Time Machine, and the other as a CCC destination. I will probably use the internal as a clone destination, and test it thoroughly before re-tasking the external clone to something else.

You certainly can do nightly backups/cloning, but for me a couple of times a week is sufficient.

Post when you make the transition and let us know how it goes! The Thunderbolt route really is the way to go, and I highly recommend any of the OWC products mentioned. I’ve purchased lots of upgrade hardware from them over the years, they know their stuff and stand behind it.

You should see a significant difference using Thunderbolt vs USB. I would not recommend going the USB route either.

Don’t waste money on all-in-one solutions when you can pick and choose better components yourself.

Probably one of the best 1TB prosumer SSDs you can get right now, $159 (2TB for $308)

And here’s a decent Thunderbolt 3 case for it with 50cm cable. $76

For less than $250 shipped, this combo will give you 2,500 MB/s throughout. Obviously, if you want to see that kind of performance over sustained periods, you’ll need an actively cooled enclosure. Or just point a fan at this one. :wink:

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