How to limit space required for Image backup to only those partitions being used Can you verify your backup version?
We do not back up unused space, so the error message is likely related to something else. Please check your TEMP location in Options (Memory Options). If there is no path explicitly set, then open a command window and type in %TEMP% to see where temp files are going. Please verify that location / drive has sufficient free space. If not, consider moving your temp location in Backup in the the Options screen.
This could also be related to retention and a new full backup. On an existing plan that has been running for some time and needs to perform a new full backup according to the schedule and retention settings, the full backup needs to complete before the old backups are removed based on retention settings. So if you had 500 GB space in backup storage, 400 GB was in use from previous backups, and the new full needed 150 GB, the backup would fail even if there were 200 GB of backups that would have been deleted after the successful backup. Not sure if this applies to you.
Can you post the exact error message as written in the History tab for that plan.