RoboCopy command wiped out new directory on destination server

Hey all,

I am just wondering if anyone could shed some light on why when I ran a Robocopy command (see below)

that it removed a folder (which was not on the source) which was created on the destination drive.

robocopy \\serverold\C:\Data  E:\Data /ZB /MIR /W:5 /SEC

so example.... Serverold  copied files to Servernew (E:) with above command, however, a folder already residing on Servernew was deleted in the process.

thanks in advance.