Not sure about the clustering part, but DataDomain has a "guide" for backing up to a CIFS share with SQL Server. Granted, the guide informs you to pass the username and password in clear text using xp_cmdshell (if DataDomain is not set up for AD authentication), but it does work. Here's what they have in their guide below. I don't have a link to the PDF or I would post it as my storage consultant emailed this to me.

Configuration and Scripting in Workgroup Mode

To communicate with a Data Domain system in workgroup mode, you need to turn

on xp_cmdshell, which is disabled by default. To do so, run the following

commands:

EXECUTE sp_configure 'show advanced options', 1

RECONFIGURE WITH OVERRIDE

GO

EXECUTE sp_configure 'xp_cmdshell', '1'

RECONFIGURE WITH OVERRIDE

GO

EXECUTE sp_configure 'show advanced options', 0

RECONFIGURE WITH OVERRIDE

GO

Example Backup Script

EXEC xp_cmdshell 'net use k: \\dd510-3\backup abc123 /user:dd510-3\sysadmin'

backup database lori_test1 to disk='k:\lori_sql_test\lori_backup.dmp'

EXEC xp_cmdshell 'net use k: /d'

Example Restore Script

ALTER DATABASE lori_test1 SET RECOVERY SIMPLE

alter database lori_test1 set single_user

drop database lori_test1

EXEC xp_cmdshell 'net use k: \\dd510-3\backup abc123 /user:dd510-3\sysadmin'

RESTORE DATABASE lori_test1 FROM DISK ='k:\lori_sql_test\lori_backup.dmp' WITH

FILE=1, NORECOVERY;

EXEC xp_cmdshell 'net use k: /d'

I personally don't use this as I am still investigating better methods. I see you posted this a while back, so you probably already figured out a workaround. If you have a better way, please share as I am not familiar with DataDomain and my company will not turn on AD authentication for it. Don't ask - they love their Novell and don't want to risk breaking it. If you ask me, it's broken upon installation.

You can also set the CIFS option in DataDomain to disable anonymous access, but that's just as bad as passing the credentials in clear text. Hope this helps.