Hi,
I was working on this last night. Sorry not to have posted the update straight away. I finished late and I was very sleepy.
The backup of sda6 has been done and then I followed your advice for the backup of sda5. It worked as a charm. Actually it would be very easy to add it to a script if you want to include support for this kind of disk. Delete sda8 after warning that a backup of the user data has to be already done, create a 5GB partition with Linux filesystem and dd all partitions there and TFTP. It works perfect.
By the way, the final size of the sda5.img.gz was only slightly over the limit 244 GB.
Well after finishing and safely store the backups I manually deleted the partitions with gdisk (I was not able to get the script to do it, but it can be that I did not choose the right options) and then I started the script choosing the options of brand new non Lacie disk witout partitions.
It created all the new partition table and started to format and so. Then I left it running and went to sleep. This morning I found it had failed. The last error said something like 'error when writting the new firmware' or something like that.
I really didn't have more time to look into it this morning since I had to run to work. I'll try again tonight and hopefully I can give some better information about what is going on.
Thanks a lot for all your support again.