Late 2015 iMac refusing to mount external bootable SSD

[bluefern]

I have a 5K 27” late 2015 iMac (iMac17,1) and the Fusion Drive died about 3 months ago. I tried all sorts of fixes (as per google advice) but nothing seemed to work. Somehow during my attempts to fix the Fusion Drive it became split into separate HDD and SSD. The SSD seemed to be cactus (it wouldn’t mount, format or anything). The internal HDD seemed fine but slow.

I ended up getting an external SSD (SanDisk Extreme Pro 4TB), installed Monterey on it and have been using it as an external boot drive. It was all working fine until a couple weeks ago when my iMac would not wake from sleep. When I restarted, it would not boot from the external drive.

I have now installed Monterey on the internal HDD (which is way slower than the external SSD) and have tried various methods to get the SSD to mount with no luck.

The SSD seems to be fine, it mounts on a PC and when I connect it to my iPad I can see all the files. However I have a gut feeling that this SSD is just too newfangled for my iMac?

On my iMac, disk utility gives me an error when trying to mount:
com.apple.DiskManagement.disenter error 49218

And running first aid fails. I tried to attach a photo but it doesn’t seem to work (probably an iPad thing). Lines from error that I feel are relevant:

warning: (oid 0x1a71b) cib: invalid o_cksum (0xfe7f00000000)
error: failed to read spaceman cib 1 at address 0x1a71b
Space manager is invalid.

Does anyone have any ideas?
Is it worth going through the pain of erasing and reformatting the external SSD and then setting it up all over again? Or will this eventually result in more hair-pulling? Would I be better off dismantling the iMac and installing an internal SSD (with all the hassle of removing the screen)? I can’t really afford to buy a new Mac.

 

[ribbo1939]

Well if it's any consolation I have the same iMac but only 10 days ago I had my fusion drive start to fail so the Apple Techs informed me, and recommended I have a new drive installed.

This I did and got it back last friday and am so pleased as it's much faster than I can recall.

The authorised techs installed a Samsung 870 EVO 2 TB SSD and I still have the original h/drive with all my docs if ever I need to look at it.

The original SSD is till in the machine as I understand its soldered to the logic board but is not connected.

I recommend you do the same. Another iMac is not on the scene that will replace it so I think I saved a lot of money by not going the mac studio & display route. The 24" is not for me.

 

[Fishrrman]

OP:

I believe you need to do this:

Boot to INTERNET recovery (this is NOT THE SAME as the recovery partition)
Command OPTION R
at boot.

Ethernet is best for this. Keep holding that key combo down for a few seconds.
If you use wifi, you'll need your wifi password. Enter it.
Be patient as the globe spins

When the utilities are loaded, open disk utility.
VERY IMPORTANT: go to the "view" menu and choose "show all devices".
Now look at the list on the left.
Pick the line that represents the physical, external SSD.
Erase it to "APFS, GUID partition format".
Quit disk utility and open the OS installer.

The installer should offer Monterey.
Start clicking through.
MAKE SURE you tell the installer to install onto the external SSD, and NOT onto the internal HDD.
The Mac will reboot one or more times, and the display will go dark for a minute or more with no other indication of activity.
BE PATIENT.

When done, you should see the initial startup screen (choose your language).
Start setup.
The setup assistant will ask if you wish to migrate from another Mac or drive.
YES, you want to do this.
So... "aim" setup assistant AT THE INTERNAL HDD, and give it time to digest everything.
Setup assistant will present you with a list of stuff to migrate. I recommend that you just accept it all.
Then let setup assistant do the migration.

When done, you should be booted from the SSD, with your stuff migrated over.
You MIGHT need to set the startup disk preference pane to pick the SSD as the boot volume.

Good luck.

 

[maclogon789]

Hi,

The Sandisk drives have a firmware issue so probably worth waiting for that resolution and potentially getting a warranty replacement if the issue cannot be fixed only with firmware (in case the bad firmware caused hardware deterioration).

SanDisk Extreme SSDs keep abruptly failing—firmware fix for only some promised

Ars saw two 2TB units become unreadable, but SanDisk only confirms 4TB troubles.

arstechnica.com

Best regards

 

[padams35]

Drive errors are never a good sign, but try an SMC and nvram reset.

Reset the SMC of your Mac - Apple Support

The SMC is responsible for managing power on your Mac. Resetting it can resolve certain uncommon issues related to power or thermal management.

support.apple.com

Reset NVRAM on your Mac - Apple Support

If one or more settings stored in NVRAM don't work as expected on an Intel-based Mac, resetting NVRAM might help.

support.apple.com

No promises, but I've seen that fix drive/partition recognition problems before (admitiedly with no error messages involved) and it is a lot less effort than attempting a wipe/reinstall.

 

[DeltaMac]

...
The original SSD is till in the machine as I understand its soldered to the logic board but is not connected.
...

Just to correct a bit of misinformation -- The SSD (a blade-type drive) is not soldered to the logic board on a 2015 27-inch iMac. It is in a slot on the back side of the logic board. Always a good challenge to get to it, but it can be removed, requiring the logic board to be pulled.
The SSD blade would be, at the most, 128GB.
If the "techs" at the service shop said they disconnected that SSD, and you don't see it in the terminal after running a "diskutil list" command, then I suspect the tech actually removed that blade, and forgot to tell you.

 

[ignatius345]

Whoof. Seeing a ton of "dead/dying Fusion Drive" threads lately. I think anyone still using one of these at this point should be very diligent about backups, and have a strategy in place to keep working when your Fusion Drive dies.

As others have pointed out, there are two points of failure (the SSD and the HDD) -- and also the tiny SSDs they used for these get absolutely thrashed with read/write cycles because of all the data being shuffled back and forth all the time.

 

[ignatius345]

Is it worth going through the pain of erasing and reformatting the external SSD and then setting it up all over again? Or will this eventually result in more hair-pulling? Would I be better off dismantling the iMac and installing an internal SSD (with all the hassle of removing the screen)? I can’t really afford to buy a new Mac.

If you're at all handy and you want to keep using the iMac, I recommend opening it up and replacing the SATA HDD with an SATA SSD. I did it a few years ago with a kit from iFixit that had tools, a bracket for using a 2.5" SSD in place of the 3.5" HDD, and some new adhesive strips. Couple hours work, most of which involved peeling off old adhesive. It's a PITA but doable if you work slowly and methodically and follow the directions. I think the whole thing, with a 2TB replacement SSD, cost me a couple hundred -- $100 for the tool and parts kit, and maybe another $100 for the SSD.

 

[maclogon789]

Hi,

I looked into replacing an internal hard drive with a SATA SSD and I don't think the small gain in speed is worth the hassle of replacing the glass, and you seem to be happy with using an external SSD currently.

Even though you have the Extreme Pro, its speed is limited to USB3 anyway, so there is no benefit to that model given your interface. If you can't wait for the updated firmware/replacement of your current drive, you could look at another external SSD. It seems like the Samsung T7 Shield is good, though the 4TB version runs slow. Also, there have been reports of thermal material leaking from the regular T7 models (not the Shield).

Some references:

Samsung Portable SSD T7 Shield 4TB Review: IP65 PSSD Gets a Capacity Upgrade

www.anandtech.com

Look at the Crystal benchmarks on page 2 and compare the Shield 2TB and 4TB models

 

[maclogon789]

Hi,

Sandisk has just posted the firmware update - note that this appears to be up as of today, and I have not seen reports of anyone trying it yet - could be worth waiting to see if there are issues.

SanDisk Extreme Portable SSD V2 (SDSSDE61) | Western Digital Product Support

With Western Digital product support, you can find an order status, register a product, check warranty status, and check on returns & replacements on your storage device.

support-en.wd.com

SanDisk SSDs, Memory Cards, and Flash Drives for Your Digital Life | SanDisk

Find detailed answers to your support questions for your SanDisk or WD_BLACK products.

support-en.wd.com

Good luck