thank you for your comment Fireball3.
I did another preclear and now there are 2 sectors pending for allocation.
The wd diagnostic tool is a good suggestion but is only windows and I do not have a windows computer where I can put the hdd in.
So if others encounter this problem, there is also the same tool in a bootable cd format. Here is the download link: http://support.wdc.com/download/dlg/Diag504cCD.iso
regards
lumpumun