Ttgjyuk

My question is very similar to this thread, but my situation is slightly different.

I have been successfully dual-booting OS X and Ubuntu for some time now with 2 partitions. I managed the partitions using OS X Disk Utility to avoid any partition table errors that Linux may incur given the new APFS structure.

Recently, I decided to shrink the OS X partition and create a 3rd partition as a dedicated swap partition for Ubuntu. Again, I managed the partitions in OS X Disk Utility to avoid errors. I successfully added the swap partition in Ubuntu (I am sure the correct partition was specified) and all seemed well. However, after I rebooted, the boot manager (stock, not rEFInd) was no longer showing OS X as a boot option. I decided to use gdisk (thank you Rod Smith for your incredible recovery tools!) to check the table structure, and it seems that the OS X partition is still recognized, but has been given the partition code FF. As far as I can tell the sectors all look good, leaving me to believe that all of my data is present. Unfortunately, I don't have a backup because my backup drive recently crapped out. Lucky me.

Can I simply change the type code of the OS X partition to rescue my data? From my understanding, APFS doesn't quite work that way.

Here is my output from gdisk -l:

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

Found valid GPT with protective MBR; using GPT.
Disk /dev/sda: 488397168 sectors, 232.9 GiB
Model: Crucial_CT250MX2
Sector size (logical/physical): 512/4096 bytes
Disk identifier (GUID): 37FE5D3B-875C-471A-B4FC-A4887DDA4659
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 488397134
Partitions will be aligned on 8-sector boundaries
Total free space is 13 sectors (6.5 KiB)

Number Start (sector) End (sector) Size Code Name
 1 40 409639 200.0 MiB EF00 EFI System Partition
 2 409640 416425263 198.4 GiB FFFF 
 3 416425264 449628383 15.8 GiB FFFF 
 4 449628384 488397127 18.5 GiB 8300 

Partition 2 is my OS X partition, partition 3 is my swap partition (not sure why its also marked FFFF instead of 8200?), and partition 4 is the Ubuntu partition.

Here is my output from swapon -s && free to prove I'm not a moron:

Filename Type Size Used Priority
/dev/sda3 partition 16601556 435456 -2

 total used free shared buff/cache available
Mem: 16335648 2089764 374952 51452 13870932 14611436
Swap: 16601556 435456 16166100

I solved my problem by following this thread's solution and using the GUID 7C3457EF-0000-11AA-AA11-00306543ECAC as my original OS X partition was an APFS container rather than an HFS partition or Logical Volume.

In summary:

1. Boot Mac in Recovery mode (Hold Cmd+R while booting)
2. Open Utilities->Terminal
3. "diskutil list" to identify the partition in question (for me, disk0s2 was marked with GUID FFFF...)
4. gpt -r show disk0" to provide the start sector and size of the disk0 partitions. The output here is very similar to the gdisk -l output of my original post above.
5. "diskutil unmountDisk disk0" to allow the next step...
6. "gpt remove -i 2 disk0" to strip the partition table data associated with the broken partition.
7. "gpt add -b 409640 -i 2 -s 416425263 -t 7C3457EF-0000-11AA-AA11-00306543ECAC"

Part 7 is the most critical step. It adds the correct data to the partition table and recovers the lost partition. Here is the breakdown of the instruction:

'-b' is the beginning sector of the partition as observed in step 4.
'-i' is the index of the partition to be recovered; in my case, this was disk0s2, so index position 2.
'-s' is the size of the partition as observed in step 4.
'-t' is the GUID type of the partition. Specifically, the originally broken data preventing use of the partition (previously marked FFFF...).