Could be three reasons.

1. Someone replaced the drive with an 800K drive.

2. Someone replaced the mainboard with a standard SE mainboard.

3. The high density disk sensor switch in the drive is bad.

The difference between the standard SE and the SE FDHD is the ROM is different 
to support high density disks and the IWM (Integrated Woz Machine) chip is 
replaced with a SWIM (Super Woz Integrated Machine) chip.

Apple sold upgrade kits with those parts, but of course discontinued a long 
time ago.

To tell the difference between a low density and high density Macintosh floppy 
drive, look in the disk slot for little vertical switch plungers at both ends 
of the slot. There should be two at the end where the write protect slide on 
the disk goes and one at the other end for detecting whether a low or high 
density disk is inserted.

If it's a bad HD switch or low density drive, replace the drive. Might be 
possible to replace the switch but compact mac 1.44M drives aren't quite that 
rare yet.

If someone swapped the mainboard or the ROM and SWIM chips then your only 
recourse is to find a FDHD mainboard or the ROM and SWIM chips from one.

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