Pour les experts, voici le fichier d'info généré par boot-repair:
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boot-repair-4ppa125                                              [20201006_2250]
============================== Boot Info Summary ===============================
 => Windows 7/8/2012 is installed in the MBR of /dev/sda.
 => No boot loader is installed in the MBR of /dev/sdb.
sda1: __________________________________________________________________________
    File system:       ntfs
    Boot sector type:  Windows 7/2008: NTFS
    Boot sector info:  No errors found in the Boot Parameter Block.
    Operating System:  
    Boot files:        
sdb1: __________________________________________________________________________
    File system:       vfat
    Boot sector type:  FAT32
    Boot sector info:  No errors found in the Boot Parameter Block.
    Operating System:  
    Boot files:        /efi/BOOT/bkpbootx64.efi /efi/BOOT/bootx64.efi 
                       /efi/BOOT/fbx64.efi /efi/BOOT/grubx64.efi 
                       /efi/BOOT/mmx64.efi /efi/ubuntu/grubx64.efi 
                       /efi/ubuntu/mmx64.efi /efi/ubuntu/shimx64.efi 
                       /efi/ubuntu/grub.cfg
sdb2: __________________________________________________________________________
    File system:       ext4
    Boot sector type:  -
    Boot sector info: 
    Operating System:  Ubuntu 20.04.1 LTS
    Boot files:        /boot/grub/grub.cfg /etc/fstab /etc/default/grub
================================ 1 OS detected =================================
OS#1:   L'OS actuellement utilisé - Ubuntu 20.04.1 LTS CurrentSession on sdb2
============================ Architecture/Host Info ============================
CPU architecture: 64-bit
BOOT_IMAGE of the installed session in use:
/boot/vmlinuz-5.4.0-48-generic root=UUID=586e3e66-e7ca-4710-a440-85b3d5e92e5c ro quiet splash vt.handoff=7
===================================== UEFI =====================================
BIOS is EFI-compatible, and is setup in EFI-mode for this installed-session.
SecureBoot disabled.
efibootmgr -v
BootCurrent: 0002
Timeout: 1 seconds
BootOrder: 0002,0000
Boot0000* Windows Boot Manager	VenHw(99e275e7-75a0-4b37-a2e6-c5385e6c00cb)WINDOWS.........x...B.C.D.O.B.J.E.C.T.=.{.9.d.e.a.8.6.2.c.-.5.c.d.d.-.4.e.7.0.-.a.c.c.1.-.f.3.2.b.3.4.4.d.4.7.9.5.}...r................
Boot0002* ubuntu	HD(1,GPT,b9a01cb1-5a09-4fba-b304-838057fe64e5,0x800,0x100000)/File(\EFI\UBUNTU\SHIMX64.EFI)
78415fb8fb9b909f8029858113f1335f   sdb1/BOOT/bkpbootx64.efi
78415fb8fb9b909f8029858113f1335f   sdb1/BOOT/bootx64.efi
2895d47544fd587b26c7e29be1295c27   sdb1/BOOT/fbx64.efi
114bd85eec32e3162958ea034020821d   sdb1/BOOT/grubx64.efi
dc3c47be2f78a78e5e57d097ae6c5c84   sdb1/BOOT/mmx64.efi
114bd85eec32e3162958ea034020821d   sdb1/ubuntu/grubx64.efi
dc3c47be2f78a78e5e57d097ae6c5c84   sdb1/ubuntu/mmx64.efi
78415fb8fb9b909f8029858113f1335f   sdb1/ubuntu/shimx64.efi
============================= Drive/Partition Info =============================
Disks info: ____________________________________________________________________
sdb	: is-GPT,	no-BIOSboot,	has---ESP, 	not-usb,	not-mmc, has-os,	2048 sectors * 512 bytes
sda	: notGPT,	no-BIOSboot,	has-noESP, 	not-usb,	not-mmc, no-os,	2048 sectors * 512 bytes
Partitions info (1/3): _________________________________________________________
sdb2	: is-os,	64, apt-get,	signed grub-pc grub-efi ,	grub2,	grub-install,	grubenv-ok,	update-grub,	farbios
sda1	: no-os,	32, nopakmgr,	no-docgrub,	nogrub,	nogrubinstall,	no-grubenv,	noupdategrub,	farbios
sdb1	: no-os,	32, nopakmgr,	no-docgrub,	nogrub,	nogrubinstall,	no-grubenv,	noupdategrub,	not-far
Partitions info (2/3): _________________________________________________________
sdb2	: isnotESP,	fstab-has-goodEFI,	no-nt,	no-winload,	no-recov-nor-hid,	no-bmgr,	notwinboot
sda1	: isnotESP,	part-has-no-fstab,	no-nt,	no-winload,	no-recov-nor-hid,	no-bmgr,	notwinboot
sdb1	: is---ESP,	part-has-no-fstab,	no-nt,	no-winload,	no-recov-nor-hid,	no-bmgr,	notwinboot
Partitions info (3/3): _________________________________________________________
sdb2	: not-sepboot,	with-boot,	fstab-without-boot,	not-sep-usr,	with--usr,	fstab-without-usr,	std-grub.d,	sdb
sda1	: not-sepboot,	no-boot,	part-has-no-fstab,	not-sep-usr,	no---usr,	part-has-no-fstab,	std-grub.d,	sda
sdb1	: not-sepboot,	no-boot,	part-has-no-fstab,	not-sep-usr,	no---usr,	part-has-no-fstab,	std-grub.d,	sdb
fdisk -l (filtered): ___________________________________________________________
Disk sda: 119.25 GiB, 128035676160 bytes, 250069680 sectors
Disk identifier: 0x7d5ab7ee
      Boot Start       End   Sectors   Size Id Type
sda1        2048 250066943 250064896 119.2G  7 HPFS/NTFS/exFAT
Disk sdb: 238.49 GiB, 256060514304 bytes, 500118192 sectors
Disk identifier: 3C572E11-3E7D-4645-AD47-C8598395CB0F
        Start       End   Sectors  Size Type
sdb1     2048   1050623   1048576  512M EFI System
sdb2  1050624 500117503 499066880  238G Linux filesystem
parted -lm (filtered): _________________________________________________________
sda:128GB:scsi:512:512:msdos:ATA SanDisk SDSSDHP1:;
1:1049kB:128GB:128GB:ntfs::;
sdb:256GB:scsi:512:512:gpt:ATA LITEON S960 256:;
1:1049kB:538MB:537MB:fat32:EFI System Partition:boot, esp;
2:538MB:256GB:256GB:ext4::;
blkid (filtered): ______________________________________________________________
NAME   FSTYPE   UUID                                 PARTUUID                             LABEL     PARTLABEL
sda                                                                                                 
└─sda1 ntfs     2A1A02001A01C9B1                     7d5ab7ee-01                          DATA ALEC 
sdb                                                                                                 
├─sdb1 vfat     1B01-4E39                            b9a01cb1-5a09-4fba-b304-838057fe64e5           EFI System Partition
└─sdb2 ext4     586e3e66-e7ca-4710-a440-85b3d5e92e5c 58714ace-ff47-46c7-b140-7eec8f49c0c6           
df (filtered): _________________________________________________________________
                  Avail Use% Mounted on
sda1              44.3G  63% /mnt/2A1A02001A01C9B1
sdb2               167G  23% /
Mount options: __________________________________________________________________
sda1              rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other,blksize=4096
sdb2              rw,relatime,errors=remount-ro
===================== sdb1/efi/ubuntu/grub.cfg (filtered) ======================
search.fs_uuid 586e3e66-e7ca-4710-a440-85b3d5e92e5c root hd1,gpt2 
set prefix=($root)'/boot/grub'
configfile $prefix/grub.cfg
====================== sdb2/boot/grub/grub.cfg (filtered) ======================
Ubuntu   586e3e66-e7ca-4710-a440-85b3d5e92e5c
Ubuntu, avec Linux 5.4.0-48-generic   586e3e66-e7ca-4710-a440-85b3d5e92e5c
Ubuntu, avec Linux 5.4.0-47-generic   586e3e66-e7ca-4710-a440-85b3d5e92e5c
### END /etc/grub.d/30_os-prober ###
UEFI Firmware Settings   uefi-firmware
### END /etc/grub.d/30_uefi-firmware ###
========================== sdb2/etc/fstab (filtered) ===========================
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/sda2 during installation
UUID=586e3e66-e7ca-4710-a440-85b3d5e92e5c /               ext4    errors=remount-ro 0       1
# /boot/efi was on /dev/sda1 during installation
/swapfile                                 none            swap    sw              0       0
/dev/disk/by-uuid/2A1A02001A01C9B1 /mnt/2A1A02001A01C9B1 auto nosuid,nodev,nofail,x-gvfs-show 0 0
UUID=1B01-4E39  /boot/efi       vfat    defaults      0       1
======================= sdb2/etc/default/grub (filtered) =======================
GRUB_DEFAULT="Options avancées pour Ubuntu>Ubuntu, avec Linux 5.4.0-47-generic"
GRUB_TIMEOUT_STYLE="hidden"
GRUB_TIMEOUT="10"
GRUB_DISTRIBUTOR="`lsb_release -i -s 2> /dev/null || echo Debian`"
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""
GRUB_SAVEDEFAULT="false"
==================== sdb2: Location of files loaded by Grub ====================
           GiB - GB             File                                 Fragment(s)
 215.131843567 = 230.996058112  boot/grub/grub.cfg                             1
 141.902477264 = 152.366624768  boot/vmlinuz                                   1
 131.355594635 = 141.041995776  boot/vmlinuz-5.4.0-47-generic                  1
 141.902477264 = 152.366624768  boot/vmlinuz-5.4.0-48-generic                  1
 131.355594635 = 141.041995776  boot/vmlinuz.old                               1
 130.984321594 = 140.643344384  boot/initrd.img                                4
  10.203346252 = 10.955759616   boot/initrd.img-5.4.0-47-generic               2
 130.984321594 = 140.643344384  boot/initrd.img-5.4.0-48-generic               4
  10.203346252 = 10.955759616   boot/initrd.img.old                            2
===================== sdb2: ls -l /etc/grub.d/ (filtered) ======================
-rwxr-xr-x 1 root root 17622 août  17 16:04 10_linux
-rwxr-xr-x 1 root root 42359 août  17 16:04 10_linux_zfs
-rwxr-xr-x 1 root root 12894 juil. 31 02:34 20_linux_xen
-rwxr-xr-x 1 root root 12059 juil. 31 02:34 30_os-prober
-rwxr-xr-x 1 root root  1424 juil. 31 02:34 30_uefi-firmware
-rwxr-xr-x 1 root root   214 juil. 31 02:34 40_custom
-rwxr-xr-x 1 root root   216 juil. 31 02:34 41_custom
drwxr-xr-x 4 root root  4096 oct.   6 22:16 backup
Suggested repair: ______________________________________________________________
The default repair of the Boot-Repair utility would reinstall the grub-efi-amd64-signed of
sdb2,
using the following options:        sdb1/boot/efi,
Additional repair would be performed: unhide-bootmenu-10s  use-standard-efi-file  restore-efi-backups  
Final advice in case of suggested repair: ______________________________________
Please do not forget to make your UEFI firmware boot on the L'OS actuellement utilisé - Ubuntu 20.04.1 LTS CurrentSession entry (sdb1/efi/****/shim****.efi (**** will be updated in the final message) file) !