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.
Drives larger than 33.8 GB will not work with kernels older than
2.2.14 / 2.3.21.
The details are as follows.
Suppose you bought a new IBM-DPTA-373420 disk with a capacity
of 66835440 sectors (34.2 GB). Pre-2.3.21 kernels will tell you
that the size is 769*16*63 = 775152 sectors (0.4 GB), which
is a bit disappointing. And giving command line parameters
hdc=4160,255,63 doesn't help at all - these are just ignored.
What happens? The routine idedisk_setup()
retrieves the geometry reported by the disk (which is
16383/16/63) and overwrites what the user specified on
the command line, so that the user data is used only
for the BIOS geometry. The routine current_capacity()
or idedisk_capacity() recomputes the cylinder number as
66835440/(16*63)=66305, but since this is stored in a short,
it becomes 769. Since lba_capacity_is_ok() destroyed id->cyls,
every following call to it will return false, so that the
disk capacity becomes 769*16*63.
For several kernels a patch is available.
A patch for 2.0.38 can be found at
ftp.kernel.org.
A patch for 2.2.12 can be found at
www.uwsg.indiana.edu
(some editing may be required to get rid of the html markup).
The 2.2.14 kernels do support these disks.
In the 2.3.* kernel series, there is support for these disks
since 2.3.21.
One can also `solve' the problem in hardware by
using a jumper to clip the size to 33.8 GB.
In many cases a
BIOS upgrade will be
required if one wants to boot from the disk.
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