It is not possible to upgrade a Macintosh SE with two floppy drives to a
Macintosh SE/30 with one SuperDrive and one 800K floppy drive. This is
because the Macintosh SE/30 logic board has only one internal floppy-drive
connector. When upgrading a dual floppy drive Macintosh SE, you must
remove one of the internal floppy drives.
It is possible to upgrade to the SuperDrive without also upgrading to the
Macintosh SE/30 logic board. In this case it would be possible to retain
both floppy drives.