Does the BIOS show it recognizes the floppy at boot-up? Sounds like it could be a weak power connection leading to the floppy. I'd recheck the connectors both power and FDC cable, both ends. It could also be 'swap floppy drive' is enabled in the BIOS. If it is turn it off. A little more info about your machine would be helpful.
Good Luck, wallie_x
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