fdisk /dev/hda
v
21292 unallocated sectors
n
No free sectors available

Why these unallocated sectors aren't available for creating a partition? Did I miss something?

A lot of this is based on the drives geomtry and block size. Standard ext block size is 512. If you do not configure each partition to stop at each block then whats left over to make the 512 block size is lost. Also there is lost between master boot sector and track 2 of the drive. I have also been told it may also be part of the drive with bad sectors as well. Not sure on that one.

Example: One Real Megabyte 1024000 divide by 512 = 2000
Example: What we think One Million is 1000000 divide by 512 = 1953.125 here you lose 512 minus 125 = 387. Next parition cannot start till 2000.
So in essences it is better to size 1 gig as 1,024,000,0000 than as 1,000,000,000

Hope I explained this correctly. This is very broad and not specfic.
Brian1

Thanks a lot. Got a point (maybe).

But there is one question:

1000000 div 512 = 1953
1000000 mod 512 = 64
We lose 512 - 64 = 448 sectors
So there is 1953 + 1 of used 64 (unused 448) sectors.
So why another partition can't begin from 1955?

It would be good for me to read more about hard drives architecture. Can you mention some good sources of information?