Update to Big Sur is stuck on progress bar.

Technically, booting from a USB installer doesn't necessarily have to wipe the internal drive with a clean installation. You can actually install on top of what you have and I would recommend trying that first. If that also fails, they do a full clean nuke and pave operation where you delete absolutely everything off the internal disk and re-install from scratch. If you have a Time Machine or other complete backup you can choose to restore it during the installation or afterwards with the Migration Assistant.

Always have a. backup.

Suggest shutdown computer and disconnect all external drives. Restart in Safe Mode using the appropriate method in the provided link.

It does a Repair Disc, clear cache files and only loads Apple Software so it loads slowly - Normal. This may or may not allow the update process to complete and present the Setup Assist for Big Sur.

Might be a good idea to burn a Big Sur installation thumb drive and boot from that and install that way. I've had much better luck doing it that way verses installing on top of an earlier release. It boots into a RAM drive with nothing running and runs the installer over the files on the disk.

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

Andy from Miramar wrote:

Restarted in safe mode, was able to go through (very slowly) the set up for Big Sur. Restarting again normally, the log in screen still shows a Mojave background, and the progress bar does not move past half way. Another restart has the progress bar barely beginning. See attach image. Any additional thoughts?

When the Setup Assist for Big Sur finished - what happened ? Were you able to Confirm Big Sur was actually installed via Apple Icon and About this Mac ?

Suggestion, again restart in Safe Mode and confirm that Big Sur was actually installed. If Not - how much Empty Space is Available on the internal drive ?

@ James Brickley has a good idea - install Big Sur from Bootable Installer. Do make a reliable Time Machine Backup Before attempting.

That same USB Installer can even be used to Wipe the Drive clean and do a real Clean Install of Big Sur. Notation - this method will strip out everything and nothing can be recovered.

@ James Brickley is totally correct and agree with upgrading over existing first. It does not require nuking the drive. That option is available as Last Resort. Plus, have a Recent TM Backup for sure.