The HDIO_GETGEO ioctl returns the number of cylinders in a short.
This means that if you have more than 65535 cylinders, the number is
truncated, and (for a typical SCSI setup with 1 MiB cylinders)
a 80 GiB disk may appear as a 16 GiB one.
Once one recognizes what the problem is, it is easily avoided.