WHY CAN'T MY NEW MAC READ THE DISKS FROM MY PLUS?  (6.2)
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       The Macintosh Plus and earlier machines along with original Mac 
  II's and some SE's do not have the high density FDHD drive necessary 
  to properly read and format high-density disks.  If you insert a 
  blank high-density disk in a low density drive, the Mac, not knowing 
  the difference between a double-density and a high-density floppy, 
  will happily format your expensive 1.4 meg disk as a cheap 800K 
  diskette.  When you move this disk to a more advanced machine 
  with a FDHD drive, the newer drive will recognize the disk as a 
  high-density floppy by its extra hole.  Since the disk has been 
  formatted as 800K instead of as 1400K, the FDHD drive will try to 
  read it as a 1400K disk and fail.  Then it will ask if you want to 
  initialize it.  As a temporary work around place a small piece of 
  tape over both sides of the extra hole on the high-density disk to 
  trick the Mac into treating the disk as double-density.  This is a 
  temporary fix only, and the tape should be removed and the disk 
  reformatted to the proper size as soon as possible.