recover data from FFFFF partition inside an external drive A few weeks ago, I deleted my Linux partition of my Mac because I was running out of space.
After that, I couldn't boot my OS anymore. Because I depend on my computer to work, I decided to backup my whole volume as it was to an external drive and format my Mac.
Now with my Mac working, once I mount my volume backup from the external drive, I can see something like below:
bash-3.2$ 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         250.8 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.8 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh HD - Data     74.4 GB    disk1s1
   2:                APFS Volume Preboot                 325.8 MB   disk1s2
   3:                APFS Volume Recovery                613.8 MB   disk1s3
   4:                APFS Volume VM                      1.1 MB     disk1s4
   5:                APFS Volume Macintosh HD            15.1 GB    disk1s5
   6:              APFS Snapshot com.apple.os.update-... 15.1 GB    disk1s5s1

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:     FDisk_partition_scheme                        *1.0 TB     disk2
   1:                  Apple_HFS Seagate Expansi         1.0 TB     disk2s1

/dev/disk3 (disk image):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        +251.0 GB   disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2: FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF               220.8 GB   disk3s2
                    (free space)                         30.0 GB    -

My question is: Is there any way of getting my data that was once in Macintosh HD ? How can I do that ??
I managed to resolve that. For those who are interested here is the solution:
Install gdisk with the following command:
brew install gdisk

After that, run gdisk passing the disk that has the FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF.
sudo gdisk /dev/disk3

Then you will be prompted with the following questions. Answer like I did below.
Command (? for help): t
Partition number (1-3): 2 // This is because the partition 2 has FFFF (see question)
Current type is 'Unknown'
Hex code or GUID (L to show codes, Enter = AF00): AF0A
Changed type of partition to 'Apple APFS'

Command (? for help): w

Final checks complete. About to write GPT data. THIS WILL OVERWRITE EXISTING
PARTITIONS!!

Do you want to proceed? (Y/N): y

After that your volume will not be FFFFFFFF-FFFF-FFFF-FFFF-FFFFFFFFFFFF anymore and you will be able to mount it and get your files back.