Mageia 5 beta 3 no longer boots

I have a Dell Inspiron 17R SE 7720 which is capable of UEFI and legacy BIOS.
My internal SSD contains Windows 7 and Mageia 4 in BIOS mode.
As soon as Mageia 5 is released, plan is to install Windows 8.1 and Mageia 5 in UEFI mode.
To have a working system while re-installing the internal SSD, I have Mageia 5 beta 3 with all updates on an external USB disk.
Partition table is GPT and install/use happened/happens in UEFI mode.
Yesterday I've put my PC back to BIOS and booted Windows 7, WHILE external USB disk was still plugget in.
Windows 7 succeeded in damaging/removing the GPT partition table, just by booting Windows 7

Once back in Mageia 4 with GParted, external USB disk seemed empty, no partitions.
Fortunately I found the tool testdisk in Mageia 4 by Googling.
With testdisk I could restore the GPT table and could access my data again.
But unfortunately I can not boot from my external USB disk.
Grub2 is not found.
How can I restore the booting of Mageia 5 beta 3 on the external USB disk?
https://wiki.mageia.org/en/EFI:_can_no_longer_boot_into_Mageia
I tried the option with the rescue function of the non-Live 64bit Mageia dvd.
But it gave an error, don't remember the exact words...
My internal SSD contains Windows 7 and Mageia 4 in BIOS mode.
As soon as Mageia 5 is released, plan is to install Windows 8.1 and Mageia 5 in UEFI mode.
To have a working system while re-installing the internal SSD, I have Mageia 5 beta 3 with all updates on an external USB disk.
Partition table is GPT and install/use happened/happens in UEFI mode.
Yesterday I've put my PC back to BIOS and booted Windows 7, WHILE external USB disk was still plugget in.
Windows 7 succeeded in damaging/removing the GPT partition table, just by booting Windows 7


Once back in Mageia 4 with GParted, external USB disk seemed empty, no partitions.
Fortunately I found the tool testdisk in Mageia 4 by Googling.
With testdisk I could restore the GPT table and could access my data again.
But unfortunately I can not boot from my external USB disk.
Grub2 is not found.
How can I restore the booting of Mageia 5 beta 3 on the external USB disk?
https://wiki.mageia.org/en/EFI:_can_no_longer_boot_into_Mageia
I tried the option with the rescue function of the non-Live 64bit Mageia dvd.
But it gave an error, don't remember the exact words...