由于仍在挂载备份、SCO保留失败-卸载失败(设备繁忙)

检查出保留错误后、显示已编制目录的备份仍被挂载：

Activity 'Registering Backup and Applying Retention' failed with error: Retention : Failed to delete backup as the backup is already mounted from primary. Please unmount the backup and retry the operation.

如果尝试删除备份仍显示为"已挂载"的资源组、则也可能会发生此问题描述。