help required after upgrade to Big Sur

Disclaimer: Big Sur will be replaced by Monterey this fall and that will make Catalina N-2 (current minus two) which means in about a year it isn't going to be supported and will stop receiving Apple security updates. Developers will stop targeting Catalina as they update their Apps.

Alternative: This model MacBook Pro Mid-2015 (MacBookPro11,4) has an upgradable SSD. You could upgrade to a larger internal SSD and then you can dual boot Catalina and Big Sur. Just create a new macOS APFS Volume and install Big Sur into one volume and Catalina into the other volume. You'll need to weight the costs as Monterey will support this model but what comes after Monterey may not. But by then much of the Mac product line should be completely switched from Intel to Apple Silicon. However, Intel systems will be supported for approximately 7 years from their release date (rough value that is not always true but true most of the time).

However, you can still downgrade to Catalina. In order to downgrade you need to wipe out the Big Sur installation from scratch and re-install Catalina then restore your data from backup. Verify you have a good backup of Catalina.

On Big Sur you can download the Catalina installer and create a Catalina bootable 16GB+ flash drive. Then boot from the flash drive, wipe the internal drive and re-install Catalina. Then restore your backup data via Time Machine or the Migration Assistant, etc.

Download Catalina while running Big Sur:

• Open Terminal and enter the following command

sudo softwareupdate --fetch-full-installer --full-installer-version 10.15.7

• Format a 16GB+ Flash drive and name it MyVolume

sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

• Shutdown completely and hold Option while powering on
• Select the USB Flash drive to boot
• Erase the internal Apple SSD with Disk Utility
• Re-install Catalina to the internal Apple SSD
• Choose to restore data during installation by selecting the backup disk or use Migration Assistant after the installation

#1 - That's false. I have cloned my internal Big Sur and Monterey SSDs to external SSDs multiple times.

The key is too erase the entire disk beforehand.

Note: if you starting with an external SSD that has a system on it Catalina or later you may have to delete the Macintosh HD, Macintosh HD -Data, Container and then the very top section. This will ensure that you get a Recovery volume on the drive.

You may have to first select "Untitled" and erase it. Then select Container Disc and erase it. Finally select the top section and erase it, making sure you have a GUID partition:

Next launch the latest version of Carbon Copy Cloner and put your internal SSD or HD as the source disk and the external SSD as the destination disk. Control (right) - click on the destination disk and select

and select "Legacy Bootable Backup Assistant…" from the menu. Next

Select Allow CCC to erase "XX"…" and click on Close. Then click the Start button.

Remember the key is in the erasure of the drive.

Carbon Copy Cloner blog regarding bootable clones:

https://bombich.com/blog/2021/05/19/beyond-bootable-backups-adapting-recovery-strategies-evolving-platform

There was a time when making a Big Sur bootable clone was a problem. Requiring re-installing Big Sur to the disk afterwards, etc. There are still some caveats to the process. It may be a problem in the future.

Yes, I know about that. I have the latest beta version as well as the latest public available version. Currently it works very well if you erase the destination disck correctly.