How to boot back into APFS container

Question

I have a early 2015 Retina MacBook Pro that was dual-booting Big Sur on its first 50GB partition and Monterey on its 2nd 200GB partition

A concern started when Disk Utility wouldn’t let me delete my 50GB partition and I read a workaround was to use diskutil on the command line so I did that and sure enough it allowed me to both delete the partition and reformat it to exFAT. Everything seemed fine for a few days and then one morning, I got the dreaded icon on startup that it couldn’t find the system partition.

The Recovery partition is lost and Internet Recovery booted and threw an error -1007F after entering my WiFi password. I found a suggestion to reset the PRAM and Internet Recovery connected over WiFi and the progress bar just gets stuck at around 40% and the ETA keeps going up and up.

So I gave up on that and went ahead and booted macOS off an external and installed Monterey to the 50GB partition and used Time Machine to restore enough to get by (partition is 25% the size so I was only able to restore so much!)

Here's what I see in diskutil right now:

% diskutil list
/dev/disk0 (internal, physical):
       #:                       TYPE NAME                    SIZE       IDENTIFIER
       0:      GUID_partition_scheme                        *251.0 GB   disk0
       1:                        EFI ⁨EFI⁩                     209.7 MB   disk0s1
       2:                 Apple_APFS ⁨Container disk1⁩         50.2 GB    disk0s2
                        (free space)                         785.0 MB   -
       3: FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF ⁨⁩              199.8 GB   disk0s3

I can't dump the GPT for some reason (or do anything with /dev/disk0 as an admin user which seems suspect):

% sudo gpt -r show disk0     
gpt show: unable to open device 'disk0': Operation not permitted

What’s interesting is when I run EaseUS Data Recovery - it can see the individual partitions (macOS and macOS Data in particular) so it doesn't seem like a 100% lost cause:

Haven't purchased a license as I'm hoping rebuilding the partition table to point back to the containers might help.

Can anyone please advise how to boot back into my 200GB APFS container?

The plan is to recover data excluded from my Time Machine backup (lesson learned), nuke it, and extend the 50GB partition and call it a day!

Answer 1

David Anderson’s answer here was immensely helpful.

The first thing I did was boot into Recovery Mode and toggle SIP in Terminal.

csrutil disable

Second, I rebooted into a OS X 10.11.6 (El Capitan) install I have on an external and logged in as admin.

I already had the gdisk utility installed and went with his advice to go this route.

sudo gdisk /dev/disk0

printing the table (p command):

Number  Start (sector)    End (sector)  Size       Code  Name
   1              40          409639   200.0 MiB   EF00  EFI System Partition
   2          411648        98527231   46.8 GiB    AF0A  Apple_HFS_Untitled_2
   3       100060432       490234711   186.0 GiB   FFFF  Untitled

before I did anything, used the b command to back up the GPT to a local file.

then used the t command to change the type of partition 3 to AF0A (APFS):

Number  Start (sector)    End (sector)  Size       Code  Name
   1              40          409639   200.0 MiB   EF00  EFI System Partition
   2          411648        98527231   46.8 GiB    AF0A  Apple_HFS_Untitled_2
   3       100060432       490234711   186.0 GiB   AF0A  Untitled

finally, use the w command to write to disk.

Rebooted holding down alt and voila, the boot manager sees both macOS instances!

Be sure to reboot back into Recovery mode and turn SIP back on.

csrutil enable

Thank you David for helping make this easy!