I can not create a backup on my external drive

Have you reported “issue” via Norton support?
Having an open Case ID: number…will get more attention.

Please review: Contact Norton Support
Note: Norton Lv1 support will request “remote session” as they’re likely to do before escalating.
If you feel comfortable with “remote session”. I’d urge you (only if you feel comfortable) to allow “remote session”. Support may have a solution. Support will likely follow they’re playbook…which likely will involve Repair Norton, Reset to Default & clean re-install. Again, only if you feel comfortable. Then “unresolved issue” can be escalated.

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fwiw ~ my boiler plate

Norton LiveUpdate + Restart (not Shut down) machine

If your Norton is not working as expected and you’ve tried Troubleshooting → Reset to Default &or Repair Norton & clean re-install Norton 360…then…reach out to Norton support.
Note: Reset to Default resets Cloud Backup

Download and run the Norton 360 Remover tool for Windows here

Maybe, try Troubleshooting → Record a performance issue &or Enable/Send debug logs

May be best to Contact Norton Support