Just to let you know I've received an explanation from Lacie, for me is useless already, because I took the disk back to the shop this evening, but maybe someone else will find it useful for the future:
Please be advised that the LaCie Setup Assistant can be used for the initial setup only and cannot be used for the second time. This is to prevent the users from using the formatting and partitioning feature (which is quite easy with this software) for the second time with the data in the drive. Once you will delete the LaCie Setup Assistant and it will not be available to download from the websites. In this case, I kindly request you to perform the DiskPart in order to resolve the issue.
Please check the number of the disk the drive is listed as in disk management.
To access Disk Management, go to your Start menu, right click on My Computer, go to Manage, and then from the left hand menu choose Disk Management. (For Windows 8, 8.1, and 10 you can simply right click on the start button and directly choose Disk Management.)
Within Disk Management, this drive should show in your list of disks in the lower portion of Disk Management (Disk 0, 1, 2, etc.). Take note of the disk number for the drive. You will need it shortly. You can leave Disk Management open because we are going to come back to it in a moment.
As an advisory statement, please understand that deleting, erasing, formatting, or partitioning your drive will permanently destroy your important data (pictures, videos, music, and documents), which Seagate cannot attempt to recover without cost to you.
1) Go to a DOS command line:
-In Windows 10/8.1/8/7/Vista, click the Windows icon and then type cmd in the search field and press Enter.
-In Windows XP, click the Start menu, select Run, then type cmd and press Enter.
2) In the command line, type:
Then, press Enter.
3) Figure out which disk you want to erase - it will be identified with a number, this is the same number I had you look up in disk management.
select disk X
“X” is the number of the disk you want to erase. This will set the focus to the specified drive.
select disk 2
Then, press Enter to erase the disk.
5) Go back to Disk Management. You should see that the disk is "Unallocated" now.
Right-click on "Not initialized" and choose "Initialize Disk".
Then, the drive should be ready to partition and format.
6) Now right-click on the "Unallocated" area.
Select "New Simple Volume or New Partition".
Agree with everything until you see this:
New Volume = (name the drive whatever you like)
Then put a checkmark in the box "Perform a quick format".
Select Apply or Start.
When the drive is ready for use it will say NTFS - Healthy.
How to Diskpart Erase/Clean a Drive Through the Command Prompt:
How to create a partition using the Disk Management to set up a Hard Drive:
Using the below link you can download the pre-included software of the drive except "LaCie Setup Assistant":?