This is not a problem but a question about hdd geometry.
What is actually the Cylinders/Heads/Sectors of a hdd?
I ran fdisk and hdparm on the same disk but they gave me different answers.
Will this affect the performance of the PC? Or even spoil the PC?

And... as you can see, I m multibooting with WinXP, FreeBSD and linux.
Is the geometry standalone on each OS? Or we must use the same values in all OSes?

Just for reference:

---------------------------------------------------------------------------
bash-2.05b# fdisk -l /dev/hda

Disk /dev/hda: 40.0 GB, 40020664320 bytes
16 heads, 63 sectors/track, 77545 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes

Device Boot Start End Blocks Id System
/dev/hda1 * 1 38760 19535008+ 7 HPFS/NTFS
/dev/hda2 38761 49152 5237190 a5 FreeBSD
/dev/hda3 49152 61073 6008310 83 Linux
/dev/hda4 61073 77536 8297572+ 5 Extended
/dev/hda5 61073 63065 1004031 82 Linux swap
/dev/hda6 63065 77536 7293478+ 83 Linux
-------------------------------------------------------------------------------
bash-2.05b# hdparm /dev/hda

/dev/hda:
multcount = 0 (off)
IO_support = 0 (default 16-bit)
unmaskirq = 0 (off)
using_dma = 0 (off)
keepsettings = 0 (off)
readonly = 0 (off)
readahead = 256 (on)
geometry = 65535/16/63, sectors = 78165360, start = 0
----------------------------------------------------------------------------------

It's a leftover from early '90s HD, actually nowdays CHS has no reason to exist, since:
1. disks have more than enought built-in logic to handle block indexes and find the place to read themselves
2. disk geometry on modern hds is far too complicated to be described by these 3 numbers
3. CHS can only handle up to 32GB (that's why you get a lower value with hdparam), and if crossed with bioses limits, will make available to bios only 2GB (hence the old booting partition limit)

Thanks for the reply.

So is it necessary to change the value of CHS in the BIOS manually or you can just leave the BIOS automatically guess whatever it thinks it is and live happily without worrying about it?

If you have a recent bios you probably won't have any problem. Once the system is booted, the os (at least modern 32bit os like linux or winxp) take care of I/O without ever calling the old 16bit bios interrupts.
My bios is dated 1996, and it's not upgradable, so I have to boot from a small partition at the beginning of the disk, but apart from that my 80GB disk works just fine under linux.