Are the backups going to disk or tape? If they are going to disk, please see the following:
Low watermark % for the magnetic mount paths was set to 0%. When jobs are sent to the storage policy, we check whether the low watermark on the magnetic mount path has been met before writing each chunk. If the low watermark has not been met, we will start writing that chunk to the mount path. Most chunks by default are 4GB in size; however, this can be verified based on the backup type in the Media Management section of the Control Panel. Since the chunks for the Virtual Server iDA backups are 4GB, the low watermark should be set to a size larger than the largest chunk that could be sent to the magnetic mount point. In this case, the low watermark should be above 4GB. This setting will allow the backup to use the other magnetic mount path when the current mount path runs out of disk space. In most cases, setting the low watermark to 9GB free space is a good start.
If this is going to tape, you can try the following:
The preferred data path option was selected so we switch it to alternate data paths when resources are offline and/or busy