Two Hard Disks Joined at the Hip – The Case of Data Recovery from an iMac Fusion Drive

Last week, we successfully recovered data from a 2015 iMac, which was using an Apple Fusion Drive. (Samsung PM830 and Seagate ST2000DM001)

The Apple Fusion Drive was first introduced by Apple in 2012 and offers lower latency rates for frequently accessed data. The Fusion Drive concept was introduced at a time when high-capacity SSDs were prohibitively expensive. Apple, always wanting to be one step ahead of the posse, believed that this new hybrid type of storage would give users a foretaste of the future.

AFusion Drive” is not the same as a “Hybrid Drive”

Using their Apple Core Storage software, two disks, a solid-state disk and mechanical platter-based one are “fused” together to create one volume. (Many users confuse a “Fusion Drive” with a “Hybrid Drive”. But a hybrid disk (such as the ST2000LX001) is different in that it uses flash memory (NAND) and mechanical platter-based storage all in one self-contained disk. On hybrid disks the flash is used as temporary storage cache, whereas with an Apple Fusion drive, data is copied (not cached) to the SSD component. Frequently accessed files can be stored on the SSD, while those less frequently used are stored on the HDD. For example, all the system files needed for MacOS to boot up are stored on the SSD meaning the iMac will boot up much faster. Users get to enjoy the speed of flash and the high-capacity of platter-based disks – the best of both worlds.

Auto-Tiered Storage is not New

Apple Fusion drive uses a form of “auto-tiering” which adaptively migrates data between the two disks. Even in 2012, it was not an entirely novel consumer-level technology. Intel’s Smart Response Technology – part of their Intel Rapid Storage Technology suite – introduced it for consumer-level computing in early 2011. Apple Core Storage acts as an LVM (logical volume manager) and migrates data between the SSD and HDD in 128KB blocks.

Diagnosing a Failed Apple Fusion Drive

This technology is great until it goes wrong of course. In this case, our customer, a professional video editor, was using a late 2013 iMac with a Fusion Drive (Samsung PM830 120GB blade SSD fused with a Seagate ST2000DM001 2TB S-ATA HDD). The Samsung SSD passed our tests with flying colours. The Seagate 2TB disk, however failed the “read” component of our diagnostic tests almost immediately. As an aside, the ST2000DM001 and its sister disks, the ST3000DM003 and ST4000DM004, have garnered much notoriety the data recovery industry. Due to their inherently unstable firmware and often weak heads, these disks can suffer all sorts of unusual ailments. Moreover, this family of disks are also known for generating all sorts of weird noises, including those of chirping, squeaking, fluttering and even scraping.

Data Recovery from Apple Fusion Drive

The ST2000DM001 disk we extracted from the iMac was no exception. It had multiple read issues and had several firmware “media cache” issues. It also made continual chirping noises akin to a caged budgerigar whose owner had hydrated it with a little too many Nespressos. Despite this, after resolving multiple issues with the disk, we managed to make a good image of it. Now, all we needed to do now was to “re-fuse” its image with the image of the Samsung PM830 SSD. This would enable us to recover the HFS+ volume. “Re-fusing” or repairing a Fusion Drive can be extremely tricky not helped by the limited repair options offered by Apple. For example, “diskutil” commands provide very limited options for manipulating Fusion Drive (Core Storage) data. And their Disk Utility tool provides no functionality for recovering a Fusion Drive.

Fortunately, at Drive Rescue we use forensic-level hardware tools which can be configured to merge two Core Storage image and reconstruct your Apple Fusion drive. For our video editor customer, we attained a 100% recovery rate of all his Adobe Premiere Pro files.

Here are a few tips for recovering from an Apple Fusion drive:

  1. Trying to reset your Fusion Drive configuration on a disk(s) which are failing can complicate issues. Refrain from doing this.  
  2. Do not attempt to re-format any disks which are part of a Fusion Drive.
  3. Do not attempt to re-install MacOS. This will overwrite Core Storage configuration data needed for data recovery.

We can help you recover data from a split iMac Fusion Drive due to physical disk problems, disk bad sectors, firmware problems and from accidental MacOS installation scenarios. We can recover from 21” and 27” iMac models such as iMac 2012, iMac 2015, iMac 2017 and iMac 2019. Successful recovery from your Apple Fusion drive means you can be re-united with previous work-related projects and of course your photos and videos