Is it possible to install Big Sur on removable media?

Yes, because that's what I do preceding any macOS upgrade. No, no particular method is necessary.

I have no explanation for the reason it's not working for you, so whatever you're doing try something different. Reformat the drive, try installing a new copy in Recovery mode, etc.

I am presently running Big Sur installed on an external SSD on my 2014 MacBook Air. You must run the installer from the Mac that the external drive is attached too, and not attempt to install Big Sur from one external drive, to another external drive via the same Mac. That dog won't hunt.

VikingOSX wrote:

You must ... not attempt to install Big Sur from one external drive, to another external drive via the same Mac. That dog won't hunt.

Challenge Accepted™

I never tried doing it that way, but you can, and it works. There is no technical reason for it not to work.

Not only were there at least two separate downloads, there were about five reboots although I may have lost count. The destination hard disk drive went to sleep for a long time, so long that I thought progress had completely halted. Any progress bars might as well have been painted on, the spinny thing went on forever, and the biggest lie in computing history must be "Less than one minute remaining".

Start to finish took probably six hours but it felt like more.

I'm using traditional rotating hard disk drives so hearing them spin up helps when I started to think it took a dirt nap. Bug Sur runs fine on them.

Hi John,

Had Mojave 10.14.6 (18G6042) installed on the internal drive of the 2014 MacBook Air and 50GB of available storage. The external Crucial 1T MX500 was the default startup drive (with hundreds of GB available storage) which had the Big Sur installer on it. The target drive was another APFS formatted 1TB Crucial MX500, so clean install. This is a 4GB RAM Air.

Boot up the Catalina drive, let fly the Big Sur installer, and pointed it at the other mounted Crucial drive. Everything proceeds normally until the first reboot, where the expected behavior was to boot from the target drive, and continue to march with the installation. Instead, it rebooted into Mojave on the internal drive, and the installation was prematurely done, and the installer on the Catalina drive had not removed itself either. This was the basis of my dogma straining at its leash…

Wound up downloading the Big Sur installer to the booted internal Mojave drive, and that produced a good, but annoyingly long installation process that was further distorted by performing the migration from a Mojave Time Machine drive. It all works well now, but worst installation experience of any Apple operating system, ever.

What you and I are doing is obviously an edge case and I am certain Apple has zero motivation to look into it. It just piqued my curiosity. I don't recommend it for any purpose beyond that.

In my case I had Bug Sur installed on an external USB 3 spinning HD. Booted from that with Startup Manager. Despite the fact it's a spinning HD, BS runs really well by the way.

I did not mount the internal Mac's startup disk. Did not even provide its FV password, so it's completely out of the picture. Then, I connected a separate, USB 2 hard disk drive with the latest Catalina release on it. Running Catalina compared to the USB 3 HDD it's rather a dog, but still completely acceptable for routine use after it starts up and everything is loaded.

Then, downloaded Big Sur and pointed to the Catalina disk for its upgrade destination. That began the arduous annoyingly long installation process you described. It may not have been the worst installation experience ever, but it's close. Multiple reboots and many useless progress indicators—as I wrote hearing occasional spinning hard disk noises helped, as was using Verbose Mode. At best it's a tie with Lion's first release.

But in the end everything worked, which is more than what I can say about Lion.

I have a screenshot that illustrates what I just described, but it's on an inaccessible volume right now. I'll post it later.

👍