Failed boot Fusion drive iMac Catalina

Continued...

You can create & use a bootable Knoppix Linux USB stick on just about any PC (even some older non-USB-C Macs). Knoppix has GNU ddrescue installed by default. The only potential issue using Knoppix is storing the GNU ddrescue log/map file especially if you end up rebooting Knoppix. While Knoppix does allow storing data on the USB stick, it may not always retain it. I would first test saving a file to the data partition on the Knoppix USB stick (while booted into Knoppix), then rebooting and confirming it is still there. I recall one time I had to use a special Knoppix boot command (aka cheat code) to retain the saved data partition.

If you don't have a PC which can boot a Knoppix USB stick or the USB to SATA Adapter, drive dock, enclosure to connect the failing drive (or don't wish to acquire any of them), then you can attempt to use GNU ddrescue from another Mac and have it scan the Fusion Drive (use the drive identifier for the Fusion Drive). I really don't recommend this option because I'm not sure if it will work due to the Fusion Drive. Plus you would need to compile GNU ddrescue or install it using Homebrew or MacPorts....all of these options will modify your host Mac & perhaps cause problems. Plus like I said before, macOS tends to get in the way here as it cannot handle the I/O errors produced by a failing hard drive. I only ever made a couple of attempts to do a recovery using macOS & gave up since it was so much easier & more reliable using Linux for the process.

FYI, if you boot Knoppix from a 2008 to 2015 non-USB-C Mac, then make sure to give Knoppix lots of time to finish booting since it may appear to be stuck on the Apple boot picker menu since the Apple firmware is very basic and is unable to display the early boot process of Knoppix.

I believe many professional data recovery services do utilize GNU ddrescue, but the professionals also have other expensive software which they can use to control & monitor the hard drive. Plus they may even pull the drive apart to replace certain internal drive components if they are causing a problem.

Keep in mind by the time you are finished, the hard drive's health will be even worse. So choose wisely on how you proceed.

Good luck.

dbfc wrote:

The drive shows up as 2 disks with Target mode, which I guess is the Fusion problem. Should I ddrescue them separately? The Disk Drill program made a clone of the disks which are .dmg files, neither of which can be mounted. Should I try to ddrescue those files first?

I don't know how Disk Drill works so I cannot provide an answer there.

If you are seeing two drives and no Fusion Drive, then the Fusion Drive is split. I don't think there is any way to rejoin them and retain the data. This will greatly reduce the chances of recovering your data since the data can be spread across both drives, although most of it should reside on the failing hard drive.

FYI, ddrescue won't rescue individual files, but instead performs a low level block for block clone. It is only after you make a complete clone (or as much as possible) that you can then utilize other tools to attempt to access the data from the cloned drive.

If you are not removing the hard drive and the Fusion Drive is split (that is you don't see that third item), then you just need to clone the failing hard drive itself. However, if the Fusion Drive is visible, then that is what you want to clone.

Which format should I use for the drive to clone the data to if using Linux?

The destination drive can have any file system or no file system or partition. ddrescue will clone the source drive making the destination drive an exact image of the source (at least for the portions of the source drive that can still be read & accessed).

Technically you can save the clone into a raw image file onto the destination drive, but then you would need yet another drive to transfer that image so that macOS & your data recovery software can read it (there is a chance you can do so from the mounted raw image file, but with macOS & macOS software this is not guaranteed to work...with Linux yes, but macOS not always). While having a copy of the cloned image in a raw image file can be handy so that you can start over, I doubt it will matter here. I will be surprised if you can clone enough of the failing drive to even attempt to recover any files with a data recovery app especially since it appears the Fusion Drive is split.

And lastly, is there any advantage pulling the disk out, rather than accessing it via Target disk mode, which shouldn't involve macOS as it isn't loaded on the source machine. I have a USB SATA adapter, though I'm not sure how reliable those things are.

Theoretically you should be able to do it through Target Disk Mode.

I guess I will have to replace the drive at some stage anyway, but it gets messy with this model of iMac with the glued on screen, and I haven't yet got a replacement drive.

You can try installing macOS to an external USB3 SSD & use the iMac by booting from the external SSD so you don't have to open the iMac to replace the internal drive. However, some hard drive failures can cause performance issues even when booting from external media.

In other words, were you using Target disk mode with your unsatisfactory attempts on Macs.

I may have tried it once a very long time ago, but I don't recall now. I have always had access to a Linux system so I have always just removed the failing hard drives as it just simplified the process & room it took. Plus either the system was going to be scrapped or was going to receive a replacement drive anyway. Sometimes I would boot the Mac with the failing hard drive from my bootable external Linux drive and clone it that way....again, it is a simpler setup involving one computer.

I never tried to clone a Fusion Drive or recover data from a split Fusion Drive. From reading this forum, people seem to have trouble getting data from a split Fusion Drive even when the hard drive does not have a severe failure. There is a good chance when you use Disk Drill that you will need to perform a deep scan....if so, you will likely end up with hundreds of thousands of files with no names. While they may be separated into file types (documents, pictures, music, video, etc.), they will be mixed in with all the temp & cache files making it very difficult to tell which ones are your actual files and which are just junk. Same thing would happen even with a professional data recovery service.

Good luck.

Looks like the Fusion Drive is split. Most times a split Fusion Drive will be caused by one of the drives failing....usually it will be the hard drive. You can try to recreate the Fusion Drive, but that will destroy all data on both internal drives. If you don't have a backup & need to recovery the data, then you will need to contact a professional data recovery service.....making sure to tell them it is for an Apple Fusion Drive.

I would definitely check the health of both internal drives. You can try running the Apple Diagnostics to see if they detect any hardware issues, but they rarely do. You can try running DriveDx (free trial period) and posting the complete text reports here so I can examine them. I don't know if DriveDx will be able to access the iMac's internal drives when using Target Disk Mode....I don't recall if I ever tried it. It is possible to create & use a bootable Linux USB stick.....I can provide instructions if you are interested if DriveDx is unable to access the iMac's drives. If DriveDx can access the iMac drives' health reports, then post the complete text reports here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar.

That hard drive is in really bad shape and it appears you may still have a lot of bad sectors pending reallocation. It is these sectors that will be causing you problems. macOS and most software (including data recovery software) are not able to handle the errors produced by a failing hard drive. The more you attempt to access the drive the worse the drive failure will get where even a professional data recovery service will be unable to recovery any data.

I've never had to attempt data recovery from a Fusion Drive before, so I'm not entirely sure of the best course of action here for attempting to retrieve data on your own. I highly recommend contacting a professional data recovery service to get a free estimate if this data is really important since you will only have one chance at retrieving it (that chance may already be gone).

If the data is not that important & you can live without it and you want to try recovering as much of it as possible, then I will provide some advice since I've successfully recovered data from hundreds of failing hard drives over the years....even ones where the drive was in bad shape (I'm not a professional, just an advanced amateur as I have no special equipment).

If I were going to attempt data recovery of this Fusion Drive, I would power off the iMac and remove the hard drive so I could attach it to a Linux system (over USB3 is fine since the I/O errors will make the recovery process slow anyway). Linux doesn't get in the way and is able to more easily ignore the errors produced by a failing hard drive.

I use the command line utility GNU ddrescue to scan the drive block by block. It is similar to "dd", but made for data recovery as it has the ability to skip difficult to read portions of a hard drive to concentrate on the areas easily read in order to recover as much data as possible. Then a second scan can dig into the more difficult to read areas dividing them into smaller groups with each additional pass. In addition it also shows its progress and the number of errors encountered. Plus it has a logging/mapping option so if you specify a log/map file, then you can easily resume the scan if it is interrupted for any reason although you must be very careful since you need to verify the source & destination identifiers have not changed since the last session. You also don't want any changes to the source & destination so it is important not to attempt to access either until you have decided to fully terminate future scan attempts.

Once you have decided you have cloned as much of the source drive as possible, then you will want to install the clone drive into the iMac and attempt to access it from Target Disk Mode. You can now attempt using First Aid or your macOS data recovery software such as Disk Drill if necessary. Without the I/O errors produced by the failing hard drive you now have your best chance at getting the data if it is at all possible to do so.

dbfc wrote:

Disk Drill purports to make a bit-for-bit copy, but it seems to save it in .dmg format. I tried to recover files from that image, and ended up with hundreds of files, which weren't very useful.

I'm guessing that Disk Drill had to perform a deep scan because the directory/folder structure is broken. When a data recovery app does a deep scan, it is looking for files identified by their file types (a hidden code within the start of the file) so there will be no file names or folder/directory structures recovered. This occurs even with a professional data recovery service. You will likely have hundreds of thousands (if not millions) of files to sort through with no easy way to distinguish the files you want from the junk. You may even encounter multiple copies of some files because the app is finding a deleted copy, or an edited copy from a temp file.

Hopefully the ddrescue image will be more effective as a source.

It will probably be the same way since you have so many pending bad blocks with so many uncorrectable errors. All ddrescue does is allow you to make a block for block image of the source as best as it can. While it may be able to recover some data from the slightly damaged areas of the drive, that data could also be corrupted.

DriveDx seemed to recognise just the one drive, so maybe the Fusion is still intact.

DriveDx should recognize the presence of any internal drives if they are communicating with the system. Usually if an internal drive is not seen, then it means that drive is not communicating or working. Even external drives should be visible within DriveDx although you may not be able to access the health information of an external drive. The USB controller used by an external drive, adapter, dock may not allow the necessary communication to access the drive's health information even when using a special USB driver. If DriveDx sees the physical drive as an option, but is unable to access the internal drive's health information, then that drive has a hardware issue.

Of course, most of this problem would have been avoided if I had a working Time Machine backup. My TM drive also failed, so I might be able to extract some data from it. It was too much to ask of a 2.5" external drive I guess. TM really flogs the drives.

2.5" hard drives have never been as robust as the 3.5" hard drives. I've rarely seen a 2.5" drive get to 80K run time hours like I have with a 3.5" hard drive. In fact, most times 5K runtime hours (maybe up to 8K) is the norm especially when installed in a laptop (an external may be about the same due to jostling of the drive & transporting them).