I look at your other post ... are you sure it has 2 Superdrives? Is the case marked FDHD? Or is it a stock SE with the Superdrive SWIM upgrade?
Typically they would not have sold an FDHD with two floppy drives and if upgraded to SWIMs, they may not have replaced both or either 800K floppy drive. Instead they may have added the after-market hard drive and bought an external Superdrive. In which case, your HD disk would not work from the iMac and might not format correctly on the 800K drive. Or it could just be bad drives ... but both of them? Suspect.