The workaround to this, is to create a CIFS mount on the device, mount the CIFS share as a Backup-2-Disk folder, and perform your synthetics there. Of course there are only a couple of select devices that can do OST and other shares simultaneously. Most only allow one protocol at a time.
However, Synthetics are limited to file based backups only, and will fail miserably on file shares with more than 50,000 files. (Note I pulled an arbitrary number, but in my experience, it just plain doesn't work at all in most environments with years of files stored)
NetBackup on the other hand is much more capable with a synthetic backup.
