growisofs has a built-in dd, which is the low-level process for sending data to the writer. This uses two buffers, the Ring Buffer (the growisofs program's software buffer) and the Unit Buffer (the drive's internal hardware buffer), and to prevent buffer underrun, these buffers must be kept filled. This is monitored and displayed in the status line as the RBU ("Ring Buffer Utilisation") and UBU ("Unit Buffer Utilisation").
^Polyakov, Andy (27 November 2003). "Re: growisofs for cd-RW and DVD-RW?". cdwrite (Mailing list). Retrieved 12 October 2011. No, growisofs does not work with nor is planned to work with CD-R[W]. My standpoint on the latter is [...]