I have a Win2k3 server with 2 instances of SQL 2005. I am unable to backup the 
default instance, but I can backup the named instance without error.
 
When I attempt to backup the default, I get the error ACO5402E - Virtual Device 
Set could not be created.
 
There is no entry in the vdi.log file on the server. And the fact that the 
named instance works, leads me to believe it is not the sqlvdi.dll as mentioned 
in some other posts regarding the Virtual Device Set error.
 
Any suggestions would be greatly appreciated.