Actually it obviously is bad because it doesn't behave reliably, but it might not be bad in a way that directly affects its ability to read disks. From your description of the behavior in which it initially refuses to acknowledge the 400/800k disk until it's been mounted in another drive *at which point it works* I would hazard a guess that there's something going on with particular drive that is breaking the usual "detect what kind of disk is inserted" routine that should happen when a disk is detected; when it's the only drive it's defaulting to 1.44 mode and giving up after that, but when you do the little detection dance it might be leaving the the SWIM in the correct mode so when you come back, viola, it works. What people are objecting to here is the contention that all drives with this model number normally behave this way; clearly this behavior is not normal and Apple never would have gotten away with selling a drive that behaved like this.
What *exactly* is broken with your individual drive is a good question; unlike normal floppy drives for PCs which are pretty "dumb" devices Macintosh drives have a rudimentary amount of stateful intelligence which is communicated via the PH0-PH3 lines. The motor behavior and head write current are completely different in the two modes of operation of the SuperDrive so there will be a register on it that needs to get flipped when it switches from one to the other; there might be documentation out there as to how exactly detection and mode switching works, reading that might clue you in on where the possible failure points lie. Off the top of my head I have no idea if SuperDrives *themselves* have enough intelligence on them to work out if there's a valid format of either type present, or if there's an elaborate dance that goes on between the SWIM and the drive every time a disk is inserted to figure it out. (It's almost certainly the latter.) It could be as simple as a bad connection on one of the PH* lines.
