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I previously installed Windows (on a partition in the main SSD, sharing it with High Sierra) on a mid 2011 21,5 inch iMac using this guide.
Afterwards I installed Big Sur using opencore legacy patcher on a secondary HDD I have on the machine. I decided to remove Windows and replace it with Big Sur, so I went ahead and tried to delete bootcamp partition using bootcamp assistant from High Sierra, it didn't detect the Windows partition so I rebooted to Big Sur and used its bootcamp assistant, the process failed with a message saying that the disk couldn't be restored or something like that, unfortunately I can't recall properly.
Since then, the High Sierra partition can't be booted/mounted/repaired and I'd like to keep my data without having to format the disk and reinstall High Sierra into it (I'm keeping a High Sierra partition because it's the last supported mac os version my machine has)
I'll leave the output of some commands that might be useful:

✘ juan@iMac-de-Juan  ~  diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS                         239.6 GB   disk0s2
                    (free space)                         260.3 GB   -

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                 Apple_APFS Container disk2         817.6 GB   disk1s2
   3:                 Apple_APFS Container disk3         182.1 GB   disk1s3
                    (free space)                         268.4 MB   -


/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +817.6 GB   disk2
                                 Physical Store disk1s2
   1:                APFS Volume Disk2                   601.3 GB   disk2s1


/dev/disk3 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +182.1 GB   disk3
                                 Physical Store disk1s3
   1:                APFS Volume Big Sur - Datos         13.9 GB    disk3s1
   2:                APFS Volume Preboot                 283.2 MB   disk3s2
   3:                APFS Volume Recovery                626.2 MB   disk3s3
   4:                APFS Volume VM                      1.1 MB     disk3s4
   5:                APFS Volume Big Sur                 32.6 GB    disk3s5
   6:              APFS Snapshot com.apple.bless.D393... 32.6 GB    disk3s5s1


juan@iMac-de-Juan  ~  sudo gpt  -vv show /dev/disk0

Password:
gpt show: /dev/disk0: mediasize=500107862016; sectorsize=512; blocks=976773168
gpt show: /dev/disk0: PMBR at sector 0
gpt show: /dev/disk0: Pri GPT at sector 1
gpt show: /dev/disk0: Sec GPT at sector 976773167
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34          6
         40     409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
     409640  468049344      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  468458984  508314151
  976773135         32         Sec GPT table
  976773167          1         Sec GPT header

 juan@iMac-de-Juan  ~  sudo diskutil verifyVolume /dev/disk0s2
Password:
Sorry, try again.
Password:
Started file system verification on disk0s2
Verifying file system
Volume is already unmounted
Performing fsck_hfs -fn -x /dev/rdisk0s2
File system check exit code is 8
Restoring the original state found as unmounted
Error: -69845: File system verify or repair failed
Underlying error: 8

Thanks in advance
edit:

✘ juan@iMac-de-Juan  ~  sudo dd if=/dev/disk0s2 bs=64 count=1 | hexdump -Cv
Password:
1+0 records in
1+0 records out
64 bytes transferred in 0.002769 secs (23113 bytes/sec)
00000000  8d 9d 91 54 9d 84 bd 1a  01 00 00 00 00 00 00 00  |...T............|
00000010  3b 56 52 00 00 00 00 00  01 00 00 80 00 00 00 00  |;VR.............|
00000020  4e 58 53 42 00 10 00 00  b8 bb 7c 03 00 00 00 00  |NXSB......|.....|
00000030  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
00000040
Juan Pablo C
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  • While booted to Big Sur, can you post the output from sudo dd if=/dev/disk0s2 bs=64 count=1 | hexdump -Cv – David Anderson Apr 30 '22 at 22:42
  • I am curious. Did you upgrade to a metal GPU before installing Big Sur? How is the video performance so far? – David Anderson Apr 30 '22 at 22:53
  • @DavidAnderson I added the output to the post, and the performance is quite good, it's pretty for daily use – Juan Pablo C May 01 '22 at 13:02
  • I didn't answer your question lol, the GPU is the default one, I followed this guide to patch hardware acceleration, it's still in beta but it's quite good https://dortania.github.io/OpenCore-Legacy-Patcher/ – Juan Pablo C May 01 '22 at 13:10

1 Answers1

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The GUID of 48465300-0000-11AA-AA11-00306543ECAC shown in your question should be 7C3457EF-0000-11AA-AA11-00306543ECAC. From Big Sur, enter the following commands to make the change. Here, I assume disk0 still identifies the drive shown in your question.

sudo gpt -f remove -i 2 disk0
sudo gpt -f add -i 2 -b 409640 -s 468049344 -t apfs disk0

 

David Anderson
  • 40,244
  • It did solve the issue, what happened exactly for the guid to change and how did you realise it should be apfs? – Juan Pablo C May 01 '22 at 21:18
  • I do not know what caused the GUID to change. By default, High Sierra installs on SSDs to APFS volumes. APFS containers have the string NXSB stored at offset 0x20. I used hexdump to confirm the existence of NXSB, and therefore concluded the GUID in the GPT was wrong. – David Anderson May 01 '22 at 21:43