Backup job fails with "insufficient storage" on the repository

Check in this order:

1. Free space on the repository volume and whether synthetic fulls need headroom (roughly 1.5× the largest full).
2. Retention — orphaned restore points from deleted jobs are a common cause; run a repository rescan.
3. Immutability windows: locked points cannot be pruned until the lock expires. This is by design and protects you from ransomware.
4. Growth rate — if the source grew, retention must be re-sized rather than trimmed on the day.

Raise a P2 if last night is the only failure; raise a P1 if backups have failed for more than 48 hours.