It results that the motherboard of my friend's computer has a bug and then the BIOS writes a copy on the hard disk and then it hides that part using HPA.

There are tools like HDAT2 that allow you to remove the HPA setting and reset the drive's space without losing anything, atleast I did not lost a file.

I'm going to write a fragment here from a blog post:

--- BLOG POST ---

Do you have a Gigabyte motherboard with Xpress Recovery BIOS?

If so, then the BIOS has truncated the drive after backing up a copy of itself to a small HPA (approximately 2000 sectors) at the top end of the drive.

GDB is detecting the correct partition size from the partition table in sector 0.

The reason the drive is reporting 33MB is that Gigabyte's BIOS has a bug that incorrectly adjusts the drive's capacity after creating the HPA. 1TB drives are reduced to 33MB, 1.5TB become 500GB, and 2TB become 1TB.

The solution is to use a tool such as HDAT2 or the HDD Capacity Restore Tool to remove the HPA (Host Protected Area).

I would also update the BIOS afterwards.

--- BLOG POST END ---

You can fully read it here:
http://forum.hddguru.com/lost-partition-hitachi-1gb-hdt721010sla360-t15662.html

And here are some other links reported by other user:
http://forums.seagate.com/t5/ATA-and-Serial-ATA-drives/1-5TB-SATA-suddenly-showing-at-500GB-and-asking-to-be-formated/m-p/52238
http://tw2005.giga-byte.com/MotherBoard/FileList/NewTech/tech_xpress3.htm
http://www.hdat2.com/hdat2_faq.html#q1 <-- The tool that I've used to fix it.

I hope this helps someone like it helped me, good luck! :)