Guide: Mac Partition Recovery; Part 1: Introduction

This guide is the result of my efforts to repair my hard drive after a partitioning error left my MacBook unbootable and all the data on the disk practically unrecoverable. It took a lot of patience, research and manual configuration but I was eventually able to fully restore my system.

Partitioning a disk enables separating System and User data and sharing a single disk between differents OS's.

Intel based Macs are compatible with most Operating Systems(OS) built for the Intel Architecture. If you connect a bootable storage device to a Mac, it will probably offer to boot it and should be able to run the OS without issue as long as you do not try to install it.

If you want to install an alternative OS on a Mac, it is important to note that this system uses a unique GUID Partition Table(GPT). One must first use Mac's "Disk Utility" to split the standard single partition scheme and allocate space for the new OS(s). The alternative OS must also be able to access disks partitioned using GPT.

I have successfully installed Windows and (if I recall correctly) Linux on my MacBook, even without the help of the "Boot Camp" facility. Both installations were relatively straightforward and my previous experience installing Operating Systems in various partition schemes afforded me the following cautions critical to avoid damaging my system:

• Do NOT alter the partition structure
• Do NOT touch the Mac partitions and avoid accessing anothre system's space
• Do have a recovery disk ready
• Do backup your working system beforehand

I usually have the first three covered but make a concession on the fourth by using a separate partition on the same disk for backups and personal data. I may have tempted fate for too long because I found myself scrambling to restore my disk to its original state after an attempt to install a different OS went awry. I instructed the installer to use the partition I set aside for system experimentation and it unexpectedly modified the Partition Table, leaving my system unbootable! Not even the built-in recovery partition was available! My Mac would light up and stall after the boot selection delay with a file icon containing a question mark appearing on screen. Total-System-Failure.

Fortunately, I could still boot from external devices, but I did not have my installation disk so my challenge was even harder, and consequently more educational. My first realisation was that GPT tools in all OS's are not yet as mature as those available for the Master Boot Record(MBR) system that was the standard in the desktop world until a few years ago.

Below is a table showing the two possible MBR partition configurations and the newer GPT system.

Intel Mac's have used GPT partitioning since their introduction and the commands available for low-level partition editing are, to great relief, available, probably owing to the FreeBSD base and UNIX certification. Unfortunately, I could not use them because I had neither recovery nor installation disks. More importantly, I needed to figure out what my installation attempt had done wrong.

"Always start at the last known point of proper operation."

This leads me to the fundamentals of recovery, especially if you are using a customised partition scheme that a single system's backup facility cannot normally restore.

• Requirements: separate recovery disk, small external disk for recording partition information, camera, calculator, utility manuals (on a separate computer with the same OS or from the internet), a bootable system disk from a unix OS with GPT capabilities.
• Do NOT attempt to boot any OS on the affected disk; use a separate recovery system. A recovery partition on the affected disk might be used, but as was the case in my experience, a corrupt partition table would render this recovery partition inaccessible.
• Backup your partition table.
  
• Do not write to the affected disk during the recovery process until all necessary information has been collected.

Continued in part2...