Reply by PurpLev

a drum sander – like a planer will only plane/sand a surface PARALLEL to the opposing surface – it will not make a surface FLAT!

IF the referenced surface is FLAT, then the parallel surface that the sander/planer will make will be FLAT as well, not because the sander/planer makes it flat – but because the referenced surface was flat to begin with.

to put it in other words – if you have a bowed/cupped board (rough cut, or a panel, or a glued up board) then the sander/planer will not flatten it – you need a jointer, or some other means (router sled) to make it flat before you use the sander.

the sander will only smoothen it out to give you a nice finish – but will not flatten it.

-- ㊍ When in doubt - There is no doubt - Go the safer route.