Hello Elmar,
Thank you for your reply.
Option 2 of the install program should have been the right choice. That keeps the boot manager data.
In fact this what I did (rewrite the loader). But at the end, I had no possibility to boot on the 1st partition and only the 2nd one with the new installed os was available within plop.
You can try the program Testdisk to recover the entry for the lost partition. In the worst case, its possible to calculate the values by hand.
I have downloaded Testdisk and followed the guide to recover the 1st partition. Unfortunately, even if Testdisk told me that the 'write' was successful, it didn't changed anything for plop. Still no 1st partition. ??
I retried it once - 2nd attempt (still no 1st partition within plop) and again - 3rd attempt (and again no 1st partition within plop).
What I don't understand here, is that usually when there is a partition change, plop display a big warning in red asking for a confirmation.
But after using Testdisk and writing the Table to the disk, there was no warning from plop. Do you have any idea about this behaviour ?I then had the idea to change the "cleared" to "don't touch" in plop and rebooted twice (experience showed me that it need reboot for plop to display values for previously "cleared" partition in 'edit mbr/import data'). I don't know if this is normal but it is what I have seen.
I then retried Testdisk.
But no 1st partition within plop...

I then by hazard tried option 2 (rewrite the loader).
This time plop displayed me values in 'edit mbr/import data'

. Houra !

So I wrote the values down

set the PB line correctly and re-set to "cleared" in place of "don't touch".
Actually PC seems ok. Both partitions boot and diskmgmt.msc display free space instead of other os.
Thank you for your help Elmar and thank you to the guy who coded Testdisk
