Backup Device

You can run INIT option with backup that will initialize the device to remove all the files, as you are taking to the same device it is not possible to drop the criteria. Simply drop the device and recreate to have full backups.

Hi Danny,
Here is the interesting discussion here on the same
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=2283786&SiteID=1 and http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=889290&SiteID=1
IMO, you can drop the device and then you can delete the physical t-log files, and, then you have an option to create a new dumpdevice for backup with same or new name.