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During my Windows 7 installation (via BootCamp) I came to screen that looks similar to this picture.

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I chose to delete and format the EFI partition (~200 MB) and and the other small partition (~629 MB). I was confused why I had those partitions in the first place so I just wanted to get rid of them. I did not delete my main Mac OS X partition though. Just the two small partitions, thinking nothing of it.

After the install of Windows 7, I realized I could no longer get back into Mac OS X. The only OS I could boot into (when I held the 'option key' at start-up) was Windows. My Mac OS X system had vanished!

I really need to save the files on my computer. I tried to research this issue with many other articles. Through many troubleshooting acts of desperation, I now can't even boot into my Windows installation.

It appears that the data should all be there. It's currently showing two partitions within my SSD. One is about 88 GB (would be my Mac OS) and the other is about 35 GB (would be my Windows OS).

Verify and Repairing the disk does not help... I have tried. I am currently accessing Terminal and Disk Utility thru the Apple Online Recovery feature.

In the Disk Utility image below, my Mac OS is 'disk0s2' and my Windows is 'disk0s4':

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Additionally, here are some terminal pictures that showcase my disk:

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  • Can you please be a bit more specific about what you have tried and how it failed? Also, embedding screenshots directly in the text makes it easier to understand what's going on here. – nohillside Oct 6 '16 at 21:29
  • Yes, I have many thumb drives if needed. It is the latest version of el capitan. and the volume is not encrypted. File vault always gave me issues in the past so I disabled it a long time ago. – craigarc303 Oct 8 '16 at 19:51
  • I AM ASKING HOW CAN I POSSIBLY REPAIR THIS PARTITION IN ORDER TO BOOT BACK INTO OS X?? I WOULD LIKE TO BOOT BACK INTO OS X SO I CAN MAKE A TIME MACHINE BACKUP... THEN WIPE THE DRIVE AND RESTORE MAC OS X WITH SAID TIME MACHINE BACKUP – craigarc303 Oct 8 '16 at 19:53
  • What would you do once you gain access? – craigarc303 Oct 8 '16 at 20:14
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I have been able to salvage at least the OS X partition by accessing the Mac remotely via TeamViewer and restoring the old CoreStorage Logical Volume Group.

Here are the steps to recover the OS X volume:

  • Boot to Internet Recovery Mode
  • Attach a 16 GB thumb drive
  • Install a full system to the thumb drive. In Internet Recovery Mode the original system the Mac was delivered with is installed: Mountain Lion
  • After booting from the thumb drive and configuring an admin account the system was updated to El Capitan.
  • wxHexEditor (a raw disk editor) and TeamViewer (remote access) were installed.

After checking the disk with disktutil list, diskutil cs listand sudo gpt -r show disk0 the following picture emerged:

  1. the disk had a MBR instead of a pMBR
  2. a CoreStorage Volume was configured but the Physical Volume was corrupted - no Logical Volume Family and Logical were available.
  3. No volume - neither OS X nor NTFS or recovery volume was working

To restore a GUID partition table the MBR had to be overwritten with

sudo dd if=/dev/zero of=/dev/disk0 bs=512 count=1

and a new GUID partition table had to be created:

sudo gpt destroy disk0
sudo gpt create -f disk0

added a new EFI partition:

sudo gpt -i 1 40 -s 409600 -t C12A7328-F81F-11D2-BA4B-00A0C93EC93B disk0

and copied the EFI of the thumb drive to disk0s1:

sudo dd if=/dev/disk1s1 of=/dev/disk0s1 bs=4m

Now I tried to determine if the main OS X partition was CoreStorage or non-CoreStorage with wxHexEditor:

In normal HFSJ volumes block 409640 and 409641 of the disk (=block one and two of the OS X partition) usually contain zeros and block 409642 the string "HFSJ" in the first 16 bytes. The first block of a CoreStorage contains some non-zeros - the next two blocks are equal to non-CoreStorage partitions.

Block 409640 had some non-zeros, so I assumed it was "corestoraged".

The end of a CoreStorage Volume Group can be determined by characteristic strings in plist-like invisible objects in the "administrative area" of the CoreStorage Volume Group.

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The string "physicalVolumes" has four occurrences near the end of an LVG. The last occurrence is in the 8185th last block of the LVG. By searching for the strings with wxHexEditor the last block was determined.

With the example above that's 511246241792/512 + 8184 = block 998536000. The block number has to be divisable by eight for 512-disks. With the last block of the LVG the second partition can be added with:

sudo gpt -i 2 409640 -s 998536000 -t 53746F72-6167-11AA-AA11-00306543ECAC disk0

The broken volume reappeared. The disk was then verified and repaired as well as the volumes. It took several attempts to repair all errors of the volume. I didn't try to recover the Windows 7 partition.

The main volume was fully bootable. All data was backed up and the drive was re-partitioned. A new system was installed and the backup data was transferred with the Migration Assistant.

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