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I have had a boot problem that I have just solved (just my stupid mistake) but by the way I tried Boot Repair which did not help (nor harm) but generated an interesting report:

boot-repair-4ppa129                                              [20210409_1530]

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


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


sda1: __________________________________________________________________________


File system:       vfat
Boot sector type:  Unknown
Boot sector info:  According to the info in the boot sector, sda1 starts 
                   at sector 0. But according to the info from fdisk, 
                   sda1 starts at sector 40.

Full report.

Another interesting bit:

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

Disks info: ____________________________________________________________________


sda : is-GPT,   no-BIOSboot,    has---ESP,  not-usb,    not-mmc, has-os,    40 sectors * 512 bytes

I have found some posts containing this message, of course related to boot problems.

One useful conclusion of my case is that the reported mismatch does not prevent booting, even though sda1 is the EFI system partition.

Nevertheless, I would like to know its cause, its possible (bad) consequences, if it needs to repaired and how to, please.

Edit after oldfred's comment

% sudo gdisk -l /dev/sda
[sudo] password for alba: 
GPT fdisk (gdisk) version 1.0.5

Partition table scan:
  MBR: hybrid
  BSD: not present
  APM: not present
  GPT: present


Found valid GPT with hybrid MBR; using GPT.
Disk /dev/sda: 1953525168 sectors, 931.5 GiB
Model: Samsung SSD 860 
Sector size (logical/physical): 512/512 bytes
Disk identifier (GUID): B5CA7AB4-D564-49A1-A43B-D9125D4DCBAB
Partition table holds up to 128 entries
Main partition table begins at sector 2 and ends at sector 33
First usable sector is 34, last usable sector is 1953525134
Partitions will be aligned on 8-sector boundaries
Total free space is 265597 sectors (129.7 MiB)


Number  Start (sector)    End (sector)  Size       Code  Name
   1              40          409639   200.0 MiB   EF00  EFI System Partition
   2          409640       975368439   464.9 GiB   AF00  MacOS_El_Capitan
   3       975630584       976900119   619.9 MiB   AB00  Recovery HD
   4       976902144      1953523711   465.7 GiB   8300  Ubuntu

And one more bit of Boot Repair report:

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

Unknown BootLoader on sda1

00000000  eb 58 90 42 53 44 20 20  34 2e 34 00 02 01 20 00  |.X.BSD  4.4... .|
00000010  02 00 00 00 00 f0 00 00  20 00 10 00 00 00 00 00  |........ .......|
00000020  00 40 06 00 4f 0c 00 00  00 00 00 00 02 00 00 00  |.@..O...........|
00000030  01 00 06 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040  00 00 29 ed 17 e3 67 45  46 49 20 20 20 20 20 20  |..)...gEFI      |
00000050  20 20 46 41 54 33 32 20  20 20 fa 31 c0 8e d0 bc  |  FAT32   .1....|
00000060  00 7c fb 8e d8 e8 00 00  5e 83 c6 19 bb 07 00 fc  |.|......^.......|
00000070  ac 84 c0 74 06 b4 0e cd  10 eb f5 30 e4 cd 16 cd  |...t.......0....|
00000080  19 0d 0a 4e 6f 6e 2d 73  79 73 74 65 6d 20 64 69  |...Non-system di|
00000090  73 6b 0d 0a 50 72 65 73  73 20 61 6e 79 20 6b 65  |sk..Press any ke|
000000a0  79 20 74 6f 20 72 65 62  6f 6f 74 0d 0a 00 00 00  |y to reboot.....|
000000b0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200
Pierre ALBARÈDE
  • 536
  • I know with NTFS Microsoft has info in PBR - partition boot sector that much match partition table. Then you normally run chkdsk on that partition from Windows to repair it. If using Windows you can mount ESP and run chkdsk on it. Or in Linux use dosfsck, from live installer, so partition is unmounted. See: https://askubuntu.com/questions/862724/grub2-failed-to-install/86587z682#865872 – oldfred Apr 09 '21 at 18:42
  • Ubuntu Disk says filesystem undamaged. I add some info in the post. There is something about 40 sectors. – Pierre ALBARÈDE Apr 09 '21 at 18:47
  • Is that after chkdsk or dosfsck? – oldfred Apr 09 '21 at 18:49
  • sudo dosfsck -V /dev/sda1 finds nothing special. (I do not have Windows.) – Pierre ALBARÈDE Apr 09 '21 at 18:59
  • May be useful: GRUB sees hd0 = first sector (not formatted). – Pierre ALBARÈDE Apr 13 '21 at 18:28
  • I have two ESP. My hexdump shows one on older HDD as "...MSWIN4" and other on newer NVMe as ".X.mkfs.fat.". You are showing ".X.BSD 4.4" as line 214. So different tools created FAT32 format. Do gdisk show structure ok? sudo gdisk -l /dev/sda – oldfred Apr 13 '21 at 20:40
  • I have completed my description according to your comment. I just realize that the 40 first sectors contain raw ASCII plain text. BSD 4.4 comes from Mac OS. The disk was probably formatted with Mac OS then repartitioned with Ubuntu. I have switched from Mac OS to Ubuntu. – Pierre ALBARÈDE Apr 13 '21 at 22:17
  • I don't have Windows, I don't need MBR. Could I simply extend the /dev/sda1 partition to start from sector 0? – Pierre ALBARÈDE Apr 13 '21 at 22:31
  • https://superuser.com/questions/1181803/hybrid-mbr-gpt-disc "The usual reason to create a hybrid MBR is, as you say, to enable a Mac to dual-boot between macOS and an older BIOS-mode OS like Windows 7 or earlier." – Pierre ALBARÈDE Apr 13 '21 at 22:36
  • The gpt partitioning normally has one MBR entry that says drive is gpt. That is to prevent old MBR only tools from seeing blank drive & trying to partition it. – oldfred Apr 14 '21 at 02:28

0 Answers0