Installing Big Sur on external hard drive

Open Disk Utility. Does it show all the volumes in the external drive?

Rereading your posts, it seems to me that what you ultimately want to do is restore Catalina to your external drive and use it as your boot drive. Am I correct?

If that is indeed what you want, then you should progress as follows:

1) Boot into Big Sur using the external drive

2) Run Time Machine to make a full backup to a third drive

3) Download the full Catalina installer using this link:

https://support.apple.com/en-us/HT211683

4) Make a bootable installer on a thumb drive

https://support.apple.com/en-us/HT201372

5) Start from the installer, run Disk Utility and completely erase the external drive - the WHOLE drive, not

just the volumes inside it.

6) Quit Disk Utility and install

7) When your mac reboots and asks if you want to migrate stuff, connect the Time Machine backup.

IMPORTANT: choose to migrate ONLY the user accounts, NOT applications, settings or other files.

Your post is confusing at best. The external drive is either a Time Machine backup or a bootable drive with Big Sur installed. Not both.

Please clarify.

Also, to start: start from your internal drive and go to System Preferences >Startup Disk and make sure that your internal drive is selected there. Then whether you start with your external drive connected or not, your mac will always boot from the internal.

Now download the diagnostic tool Etrecheck and post its full report here using the “additional text” button below your message area. This will let us know more about your system.

Also: select the external drive jn the Finder and press Command-I to open the Info window, and post a screenshot.

The drive you are showing is NOT a Time Machine backup.

It is only one volume that is part of what is usually in a Big Sur drive.

I will have to look at the Etrecheck report but it looks like your older OS may not be recognizing the full structure of the drive.

Ok, so you are running Catalina, which should recognize a properly formatted disk with a Big Sur installation. I will have to read the report carefully (not so easy on my iPhone screen). I will write back later today.

> Catalina, which should recognize a properly formatted disk with a Big Sur installation

I guess both Mojave and Catalina show the Big Sur system-volume confusingly as “Update”.

"persistent hidden volume called “Update,” and it is visible if you boot into Catalina on the same system)"

https://arstechnica.com/gadgets/2020/11/macos-11-0-big-sur-the-ars-technica-review/11/#h3

Can you post a screenshot of what you see in Disk Utility? We need to know exactly what volumes are there in the external drive, in order to figure out which one does contain your data. Once we do that, you will have to copy the data elsewhere.