USB storage (thumb drives, SDs, TB-2 drive) connected to internal Apple devices can no longer be mounted.

No. I've stopped updating as of version 11.4. Rumor has it that my printer/scanner may no longer respond after updating to 11.5.1. Are you suggesting that Apple made some improvements in this latest version that will help with the USB issues?

After the authorised repair service backed up the SSD and re-installed Big Sur (re-installing the files and applications from their back up), all USB ports (incl. TB-2) began working again. I was able to repeat the backup in TimeMachine and clean re-install Big Sur, successfully. Restoring my TM-backup helped ensure that my own clean install of the MacOS couldn't be compromised by older versions of items that had been grandfathered through over various years of system upgrades. Thus, the upgrade to Big Sur proved to be the least forgiving, i.e., the most error-prone in comparison to all previous upgrades, beginning with my upgrade to Snow Leopard.

Ergo: Everyone ought to best be prepared in future to first flatten the system (after backing up!) and then clean-install newer OS versions, using a full TimeMachine backup to restore user files. Fallback: Keep a clone of the last running system ready in case any upgrade leads to new unexpected issues. It's no fun to lose USB-device service and have to struggle with recovery mode to get things working again .

Thank you for your reply, but yes, I had seen and folowed that suggestion earlier, with no positve result. I have also tried Apple Diagnostics; it returned the code that all was Okay.

Additional text not available on my iPhone - the MacBook has gone in for data back up and re-install of Big Sur. I can send that report if the system repair doesn’t help with the USB issue . Thanks!