Some times it happens that it says "all erase" don't do it.what you sometimes can do is move that track to the end and attempt to erase it from there.
I've got a mz-G750 so I suppose it will be similar.
When you want to erase a particular track .. start playing it ..then press 'enter' it should then display "edit".. press 'enter' ..it then displays "name" press >> "move" press >> "Erase" is then displayed at this point press 'enter' and the track should be erased.
Oh yes, remember not to have the disc protected !!
all the very best ritchie.