Hello,
Refer to the following technotes that explain the procedure to create removable backup-to disk folders:
Title: Backup Exec support of USB and Firewire hard drives for backup-to-disk in a Windows Server environment
http://support.veritas.com/docs/276904
How to create a new removable Backup-to-Disk folder in Backup Exec 9.x and 10.x for Windows Servers
http://support.veritas.com/docs/253764
Also check the following:
Best Practices for using removable media and the "Backup to Disk" feature in Backup Exec for Windows Servers
http://support.veritas.com/docs/239539
In case you have any more questions on this issue, revert with details.
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Regards.