My first piece of advice would be to confirm the version of the Backup Exec product you are using supports removable/external disks. If not, it may not be possible to resolve the quirky behaviour.
The second step is to note whether any telltale events appear in the system event logs or the Backup Exec application log.
Removable drives cause headaches to users as well as applications. You may have spent many an hour reassigning drives in Windows after letters changed from those they were originally assigned. The behaviour could well be an effect of the operating system (OS) in which Backup Exec is unable to verify/query the status of the backup target device.
One possible solution would be to mount the external disk as a mount point and not a drive letter, in this way stopping the OS from changing the external disk drive letter.
Prior to scheduled backup confirm the following:
- The external disk is mounted to the correct OS drive letter/location.
- The backup target device (configured on the external disk, based on its OS drive letter/location) is available in the backup application.
- If not, does a stop/start of the backup services resolve the issue?
- Is the backup target device protected by a retention period?
- Does the manually initiated backup differ in retention and attributes to the scheduled backup (overwrite protection as an example)?
If all else fails and you have a support contract with Symantec, don't be afraid of logging a call.
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