_**Work In Progress. Don't follow this solution until this notification is gone**_
The method to repair your disk and recover the GUID partition table is related to my answers to similar questions: [HFS+ invalid number of allocation blocks][1] and [Hard drive no longer accessible][2].
Basically you have to find characteristic strings of JHFS+ volumes, use some simple math and common sense and have some luck to fix the GUID.
Additionally there are some fixed sizes and rules that help you to determine some of the sizes as well as some of the starting and the ending blocks of your partitions.
1. 1st block (block 0) = PMBR
2. 2nd block (block 1) = Pri GPT header
3. 3rd - 34th block (block 2 - block 33) = Pri GPT table
4. 41st - 409640th block (block 40 - block 409639) = EFI (aligned)
5. 409641st - ??? block (block 409640 - block ???) = partition 1 (aligned)
6. empty space 262144 blocks (aligned)
7. ??? - ??? block (block ??? - block ???) = partition 2 (aligned)
8. empty space 262144 blocks (aligned)
9. 7 empty blocks to keep alignment
10. the last 33 blocks except the very last one = Sec GPT table
11. last block = Sec GPT header
12. alignment rule: the start block and the sizes of all partitions (EFI, partition 1 & 2) and the major empty spaces is dividable by 8
Finally a `gpt` command should result in something similar like this output:
root# gpt -r -vv show /dev/disk1
gpt show: /dev/disk0: mediasize=3000592498688; sectorsize=512; blocks=5860532224
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 5860532223
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 2930844728 2 GPT part - 48465300-0000-11AA-AA11-00306543ECAC
2931254368 262144
2931516512 2928753528 3 GPT part - 48465300-0000-11AA-AA11-00306543ECAC
5860270040 262151
5860532191 32 Sec GPT table
5860532223 1 Sec GPT header
*Hint: Since I can't create a disk of the same size as yours in Parallel Desktop some sizes are different than your original sizes*
**Preparation:**
Backup your Mac and then detach all external drives except the one you want to recover.
Download and install [wxHexEditor][3]. Enable the root user and log-in as root.
*Hint: While working with wxHexEditor don't use copy and paste. Enter everything manually! You might accidentally write directly to your disk.*
**Determine your partitions` boundaries:**
Open the Calculator. Open wxHexEditor. Check that you work in read-only mode ("Options" -> "File mode" -> "Read only"). In the menubar go to "Devices" -> "Open disk device" -> choose the appropriate diskNumber. Probably it's disk1. The disk should have one partition (disk1s1). Please try to arrange the wxHexEditor window like in the examples below with straight red lines.
Then hit the "Go to offset"-button (marked with the green circle) and enter 409640 exactly like in the picture below. Sometimes you have do that twice to jump to the correct sector. Re-check the correct sector by entering the offset (marked red) in the Calculator and divide it through 512.
[![enter image description here][4]][4]
If you see a similar picture you already found the beginning of your first partition (note the string HFSJ in block 409642!).
Now jump to the midst of your disk: Hit the "Go to offset"-button and enter block number (total blocks of your disk/2) ~ 2930266108.
_This works if you've previously partitioned your disk with Disk Utility by simple choosing 2 partitions in the drop down menu. If you've adjusted the slider between the two partitions afterwards e.g. enlarged partition1 you have to jump to slightly bigger offset._
Now hit the "Find"-button (marked with the green circle) and enter HFSJ like in the picture below and hit find. This may take a while.
[![enter image description here][5]][5]
If the search was successful you've found the beginning of the second partition. Make a note of the block offset (=***BlockOffset2***). In my example the offset is 1500936455168. If you scroll to lower offset numbers the disk should be filled with 0s.
Since you have found the start sectors of both volumes and the rest usually is determined by the rules 1-12 mentioned earlier you may now fix the GUID table.
Quit wxHexEditor. If you are asked to save changes don't save them!.
---
**Now some you have to do some math:**
The first HFSJ string usually is found in the 3rd block of a HFSJ volume.
So the first HFSJ volume starts at block 409640 (also rule 5).
The second HFSJ volume starts at ***StartBlockOfVolume2*** = ***BlockOffset2***/512 - 2. In my example that's 1500936455168/512 -2 = 2931516514 -2 = 2931516512.
With the start block of volume 2 and the fixed empty space of rule 6 you may determine the end block of volume 1:
First block of volume 2 - 262144 (rule 6) - 1 = ***EndBlockOfVolume1***.
In my example that's 2931516512 - 262144 - 1 = 2931254367
***SizeOfVolume1*** = ***EndBlockOfVolume1*** - start block volume 1 (rule 5) + 1
In my example that's 2931254367 - 409640 + 1 = 2930844728
The only thing we need missing is the size of volume 2:
With the rules 8-11 from above you may now determine the last block of volume 2:
Total size of disk in blocks - 1 (rule 11) - 32 (rule 10) -7 (rule 9) - 262144 (rule 8) - 1 = ***LastBlockOfVolume2***
***SizeOfVolume2*** = ***LastBlockOfVolume2*** - ***StartBlockOfVolume2*** +1
---
**Rebuild a proper GPT:**
Here i assume the identifier of your external disk is disk1.
First you have to unmount your external disk:
diskutil umountDisk disk1
Check the partition layout and remove the only partition with fdisk or gpt:
gpt destroy /dev/disk1
First we rebuild the EFI with:
gpt add -b 40 -i 1 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B disk1
Then we add the first partition:
gpt add -b 409640 -i 2 -s SizeOfVolume1 -t 48465300-0000-11AA-AA11-00306543ECAC disk1
Then we add the second partition:
gpt add -b StartBlockOfVolume2 -i 2 -s SizeOfVolume2 -t 48465300-0000-11AA-AA11-00306543ECAC disk1
[1]: http://apple.stackexchange.com/questions/176777/hfs-invalid-number-of-allocation-blocks
[2]: http://apple.stackexchange.com/questions/177553/hard-drive-no-longer-accessible
[3]: http://sourceforge.net/projects/wxhexeditor/
[4]: https://i.sstatic.net/JD7La.png
[5]: https://i.sstatic.net/eYnHu.png