Open the case and check the ribbon cable and power supply cable going to the floppy drive. Make sure both are fully seated, and the ribbon cable is not one pin off. If the floppy LED stays on all the time, flip the ribbon cable over. Check the floppy connection to the motherboard too, be sure it's fully seated and not one pin off.
Sounds like you either didn't replace a cable after installing the new drive, knocked one loose, or got it plugged in upside down.
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