Pre-exec batch-file contains command needing runas, but doesn't work?

DP A.09 on Windows, but I'm a newbie . .

Backing up a Universe db on Windows Server 2003, and previous backup system has a pre-exec batch-file that closes down Universe and runs SYNC (from Sysinternals) to flush the disk cache.  That seems to work OK.  But SYNC needs to be Runas administrator.

Now trying to reproduce this in DP - put the batch-file into the Filesystem Options box, and changed the SYNC D line in the batch-file, to: runas /user:domainadmin@domain /savecred "cmd.exe /c SYNC D > C:\batch\output.txt".  The DP job starts OK & Universe stops, but there is no output file produced (it should contain "Flushing: D" to confirm it ran OK, and when I know it works I'll remove the redirect).  Outside the DP environment, the entire batch-file runs OK and the output file is produced correctly.

Any ideas, anyone?

TIA

Ian