When on parade with the band, the drum major may or may not hold a rank, but will assume command by the appointment of Drum major.
if the D/M is a Sgt then the badge shown is that of 4 chevrons only
if the D/M is a C/Sgt then the badge is as above with the C/Sgt crown
if the D/M is a WO2 then the badge is as above with the wo2 crown.
So to summerize: Drum/bugle/pipe Major is an appointment not a rank.