When setting up Backup Exec to save to an USB or eSATA connected storage device, use the following best practices.
Never save the backup job to the root of the device. This will cause the backup job to sporadically fail. Always create a folder at the root of the device and configure Backup Exec to save to that folder.
When setting up your backup job, always create an inventory job to run before the backup job. This will prevent the backup job from failing when it verifies.