I know this is an old post, but I’m going to respond anyway, because someone else might find this info useful and applicable in 2022. I have tried Data Rescue in the past with great results if I recall correctly. But, GetMyDataBack has been the fastest, easiest, and most accurate recovery tool I that I have come across, and it was free back then, not sure how much now.
Before you get all excited and start trying all types of recovery software on your drive, you should know that you must avoid using the corrupted drive in order to save what is left of the data without causing more problems. The more you use the drive, the more likely that you will cause additional damage to the data and the file system.
The best thing you can do to save your files and limit potential loss of data, is to create an identical clone or disk image to backup your drive. Then, work on repairing the disk image instead of repairing the physical drive.
Based on my experience in the IT field for years, the majority of drive failures that I have seen were related to corruption of the drive’s partition table, not the actual file system. In other words, there is a great chance that your data is still intact, because partition tables, in simple terms, is like a map to that helps guide your system to access your files. Losing the map to your destination doesn’t mean that the physical location was also “lost”, you just lost your way to to get to your destination thats all. Repairing the partition table is similar to repairing or finding a new map to your destination.
You can rebuild the partition table using basic terminal commands in macOS, Linux, or even Windows, and can usually accomplish in less than an hour. Just research how to rebuild GPT, MBR partitions as a head start.
More Technical explanation:
Partitions are contiguous sections of a hard disk, identified by sector numbers, such as sectors 2,048 to 41,943,040. Partitions are defined in a data structure called a partition table, which is simply a list of sector ranges and some simple associated data (such as a type code to identify the type of data the partition contains). Two common partition table types are MBR and GPT.
Filesystems are much more complex data structures that enable storing, organizing, and locating individual files on the disk. Filesystems are often stored inside partitions, which simply means that they occupy the range of sectors identified by the partition table as being a particular partition. Numerous different filesystems are available, such as FAT, NTFS, HFS+, ext4fs, and so on.
Thanks to klanomath from stack exchange, here is one of his examples on how to repair GPT partition on macOS. Don’t try this without first investigating the problem associated with your particular drive.
Only use this if you know what your doing:
To correct the GUID error in your GPT, you need to start up from OS X Recovery over the Internet or from a USB drive with the macOS Installer flashed on it. To boot into recovery mode press and hold the command(⌘)optionR keys immediately after you turn on your Mac and hear the startup sound. To boot into a USB key, hold option immediately after you turn on your Mac. You will be prompted to select a drive, one of which will be titled something similar to “macOS Installer” which you will need to select. Once started to OS X Recovery or the Installer, select the Utilities Menu, then click on the Terminal application from the menu bar. Enter the following commands one at a time and press enter after each command to repair your GPT.
diskutil unmountdisk /dev/disk0
gpt remove -i 2 /dev/disk0
diskutil unmountdisk /dev/disk0
gpt add -i 2 -b 409640 -s 194016208 -t hfs /dev/disk0