Ubuntu 18.04无法启动，Boot Repair提示无已知引导加载器安装的修复求助

我在单系统（无双启动）的设备上安装了Ubuntu 18.04，强制重启后无法启动，进入了grub_rescue提示符。

在grub_rescue提示符下执行ls命令，显示的分区列表为：

(hd0) (hd0,msdos7) (hd0,msdos6) (hd0,msdos5) (hd0,msdos3) (hd0,msdos1)
其中(hd0,msdos7)和(hd0,msdos1)是ext2文件系统，其他分区显示为未知文件系统。

我按照论坛上的解决方案尝试设置路径和前缀等，但问题没有解决。现在在grub_rescue下执行ls只能看到(hd0) (hd0,msdos7)，甚至在fdisk -l中也看不到我的分区了。

我在Live会话中运行Boot Repair，得到以下输出：

boot-repair-4ppa203                                              [20230427_1339]

============================== Boot Info Summary ===============================

=> No known boot loader is installed in the MBR of /dev/sda.

sdb: ___________________________________________________________________________

File system:       iso9660

Boot sector type:  Unknown

Boot sector info:

Operating System:

Boot files:        /boot/grub/grub.cfg

================================ 0 OS detected =================================

================================ Host/Hardware =================================

CPU architecture: 64-bit

Video: Xeon E3-1200 v2/3rd Gen Core processor Graphics Controller from Intel Corporation

Live-session OS is Ubuntu 64-bit (Ubuntu 18.04.6 LTS, bionic, x86_64)

===================================== UEFI =====================================

BIOS/UEFI firmware: 4.6.5 from American Megatrends Inc.

This live-session is in Legacy/BIOS/CSM mode (not in EFI mode).

============================= Drive/Partition Info =============================

Disks info: ____________________________________________________________________

Partitions info (1/3): _________________________________________________________

Partitions info (2/3): _________________________________________________________

Partitions info (3/3): _________________________________________________________

fdisk -l (filtered): ___________________________________________________________

Disk sdb: 28.7 GiB, 30752000000 bytes, 60062500 sectors

Disk identifier: 0x4c050c47

Boot   Start     End Sectors  Size Id Type

sdb1  *          0 4910399 4910400  2.3G  0 Empty

sdb2       4840192 4844991    4800  2.4M ef EFI (FAT-12/16/32)

parted -lm (filtered): _________________________________________________________

sda:500GB:scsi:512:4096:unknown:ATA ST500DM002-1BD14:;

sdb:30.8GB:scsi:512:512:unknown:SanDisk Ultra:;

blkid (filtered): ______________________________________________________________

NAME   FSTYPE   UUID                                 PARTUUID                             LABEL                    PARTLABEL

sda

├─sda1

├─sda2

├─sda5 swap     b86ba29a-dd33-44f6-878d-9d2f507ff74d

├─sda6

└─sda7

sdb    iso9660  2021-09-15-20-41-59-00                                                    Ubuntu 18.04.6 LTS amd64

├─sdb1 iso9660  2021-09-15-20-41-59-00               4c050c47-01                          Ubuntu 18.04.6 LTS amd64

└─sdb2 vfat     7E74-30B0                            4c050c47-02                          Ubuntu 18.04.6 LTS amd64

Mount points (filtered): _______________________________________________________

Avail Use% Mounted on

/dev/sdb                   0 100% /cdrom

Mount options (filtered): ______________________________________________________

====================== sdb/boot/grub/grub.cfg (filtered) =======================

Try Ubuntu without installing

Install Ubuntu

OEM install (for manufacturers)

Check disc for defects

==================== sdb: Location of files loaded by Grub =====================

GiB - GB             File                                 Fragment(s)

?? = ??             boot/grub/grub.cfg                             1

======================== Unknown MBRs/Boot Sectors/etc =========================

Unknown MBR on /dev/sda

Unknown BootLoader on sdb

Suggested repair: ______________________________________________________________

The default repair of the Boot-Repair utility would not act on the boot.

有没有办法不用重装系统就能修复这个问题？非常感谢任何帮助。

备注：内容来源于stack exchange，提问作者Jayant