hd probleme on fedora

[priou]

hi, 


i have 2 hd on my computeur :
/dev/hda 40go ( linux systeme partition ext3 ) 
/dev/hdb 80go ( linux partition ext3 ) 


 my /dev/hdb have four principal partition  ( 4x20go) 

now i would like 2 partions, but i try : fdisk /dev/hdb 

- delette all partition 
- write info on Hd 

after i ve rebooted my computer  and i run another time : fdisk /dev/hdb 
 - create partition
 
but fdisk read my old partition and allocate only 20go, why ? 

i try after created my new partition 20go to mount this on my system, but this 
partition is not empty .... :( 


on linux, how to make for broken my mbr of this HD ? 
or 
how to make for to have  2 partitions  instead of 4 on my HD ? 



thank for your answer 

priou alexandre