DD versus HD 5.25" floppy drives

I am assumign that you are going to use the HD drive in DD mode, that is at the lower write current. In which case the difference is the number of cylinders, and thus that the 80 cylinder 'HD' drive writes a narrower track than a true 40 cylinder 'DD' one. The rules is that if you write with an 80 cylinder drive to a disk that was previosuly written ona 40 cylinder drive (this includes formatting in both cases), then there could be problems reading it on a 40 cylinder drive, because the narrower head of the 80 cylidner drive will only rewrite the middle part of the track, so the wider 40 cylidner head will see a mix of the old and new data. So : Bulk erase a disk, format it o nthe 80 cylidner drive, write to it on the 80 cylinder drive and it'll be readable on a 40 cylinder drive. You can even write to it there without prolbmes. But if you do write ot in o nthe 40 cylinder drive, do not then write to it on an 80 cylinder drive. -tony