Data Recovery from inaccessible Samsung Evo 750 SSD
This Samsung Evo 750 SSD (MZ-750500) taken from a 2013 Apple iMac was no longer accessible to the user. Instead, this retro-fitted disk, presented him with the dreaded “flashing folder and question mark” screen. When in Apple’s Recovery Mode, the disk also failed to appear in Disk Utility. The user, an author, stored copious amounts of PDF, Word and image files on it – all of which needed to be retrieved to meet upcoming publication deadlines.
The Samsung Evo 750 is an SSD, which was introduced by the company in 2016. It uses 16nm planar-based TLC NAND, has a 512 DRAM cache and is managed by an MGX (Samsung in-house) controller.
Our SSD data recovery equipment provided us partial access to the Evo’s firmware. The wear- level of the SSD’s blocks was high and it also became apparent that the disk was over 94% full. An almost full-capacity solid-state with high-wear levels is far from optimal. This is because when some blocks go bad, the SSD controller will allocate (good) spare blocks to replace them with. This is known as bad block management (BBM). But, here is the catch, when the controller has a sparse level of blocks to choose from and those that are available are worn out – the controller can easily lock-up. We suspect this is why the user could no longer access his data.
How could this have been prevented?
First and foremost, this problem could have been prevented if the user didn’t fill his disk up to near-full capacity. It’s generally a bad idea to use an SSD that is over 90% full, without freeing up some capacity first. SSDs need some breathing space to perform essential housekeeping operations like garbage collection and BBM.
Secondly, the user might have been prevented the problem if they over-provisioned the disk. Over-provisioning can be achieved by creating a partition that does not use the disk’s full capacity. This “unclaimed” space will then be used by the SSD controller as a pool of “spare” blocks.
In some cases, SSD manufacturers already do this in a process known as “factory over provisioning”. But, for this disk, Samsung did not factory-over-provision it. The deployment of factory over-provisioning is usually indicated by the disk having an “uneven” capacity. For example, a 480GB disk is normally a 500GB disk, but with 20GB reserved for over-provisioning. Likewise, you can have a 960GB SSD, which is really a 1TB disk with 40GB allocated for over-provisioning.
The paucity of free blocks was not the only factor which culminated in this disk becoming inaccessible. Consistent with many earlier generations of SSD disks, this Samsung 750 Evo SSD was only using 2D planar NAND. This suffers higher rates of cell-to-cell interference than 3D NAND commonly used in SSD’s today.
Recovering the Samsung Evo 750 with a Firmware Emulator
Even though the controller on the disk appeared to be locked. We were able to use a firmware emulator to access the partition table. The emulator mimics the disk’s MGX controller, enabling us to get access to its APFS partition. Much to his satisfaction, the author got all his files retrieved and was saved the painful process of re-doing work which he had already completed. Moreover, he would meet all of his publication deadlines.
Drive Rescue is based in Dublin, Ireland. We offer a SSD data recovery service for inaccessible SSDs such as the Samsung Evo 750 (MZ-750500, MZ750250), Samsung Evo 840, Samsung Evo 850 (MZ-75E1T0) and Samsung Evo 860 (MZ-76E500BW and MZ-76E1T0b). Contact us on 1890 571 571.