Diagnose failure, Fusion Drive or motherboard issue

[mich8261]

I have a late 2015 iMac with Fusion Drive. A few weeks ago it booted up to disk utility showing the main drive wasn’t mounting. Now it doesn’t even get to this point. It goes into some kind of safe made, saying “checking drives” with a spinning wheel, but it just stays like that for hours.

Using another Mac I loaded macOS Monterey on an external SSD. I connected it to the iMac (usb 3 cable) and was able to boot up. I thought my problem was solved but then I noticed that just about any task grinds to a halt. For instance I tried to pair a Bluetooth Magic Mouse and that crashed the Settings app. Also, the only drive that shows up is the one running the OS. I have external drives with Time Machine and photos connected (via a 4-port USB 3 hub). It none of these are showing up.

Sorry for the long intro, here is the question, how can I confirm that the issue is actually the Fusion Drive and not the motherboard? I am prepared to replace the Fusion Drive with an internal SSD, but I want to be sure this is the issue. Thank you

 

[ignatius345]

Someone else with more experience would have to weigh in, but I believe that if you're booted off an external drive you should basically be bypassing the internal. So that would seem to indicate a bigger problem.

Can you run DriveDx from the external drive? It'll work for your purposes in demo mode, if it'll run.

 

[Nguyen Duc Hieu]

Sorry for the long intro, here is the question, how can I confirm that the issue is actually the Fusion Drive and not the motherboard? I am prepared to replace the Fusion Drive with an internal SSD, but I want to be sure this is the issue. Thank you

Very easy.
Physically remove the Fusion Drive and all other peripheral devices like the Time Machine drive from your iMac and boot it again with the external drive.
If everythings run smoothly, you know the cause.
If not, yeah, you already know the result.

 

[Yebubbleman]

I have a late 2015 iMac with Fusion Drive. A few weeks ago it booted up to disk utility showing the main drive wasn’t mounting. Now it doesn’t even get to this point. It goes into some kind of safe made, saying “checking drives” with a spinning wheel, but it just stays like that for hours.

Using another Mac I loaded macOS Monterey on an external SSD. I connected it to the iMac (usb 3 cable) and was able to boot up. I thought my problem was solved but then I noticed that just about any task grinds to a halt. For instance I tried to pair a Bluetooth Magic Mouse and that crashed the Settings app. Also, the only drive that shows up is the one running the OS. I have external drives with Time Machine and photos connected (via a 4-port USB 3 hub). It none of these are showing up.

Sorry for the long intro, here is the question, how can I confirm that the issue is actually the Fusion Drive and not the motherboard? I am prepared to replace the Fusion Drive with an internal SSD, but I want to be sure this is the issue. Thank you

View attachment 2150656

Fusion Drives are tricky. You ought to be able to run a drive check on each individual drive/device when booted from an external drive.

In general, my advice as far as Fusion drives is this: the hard drive is a ticking time bomb and if your Mac is out of warranty, I recommend taking it to an Apple Authorized Service Provider (and NOT an Apple Retail Store's Genius Bar) with a set of replacement drives (I like OWC's drives, but many people on here will disagree with that; just make sure you have drives that will work in your iMac) and have them do the replacement. If the drives are large enough, then you don't need to rebuild them into another Fusion Drive; you can just have an iMac with two decent and capacious SSDs.

All that to say that, if you have a good Time Machine backup and are not already looking toward replacing this iMac with something newer (which you might want to do, considering your 2015 iMac can't run newer than macOS Monterey and since that gives it a one and a half year shelf-life as a Mac), you might want to just have whomever replace the drives anyway. In the process of doing so, a tech can verify to you if there's a problem outside of those drives.

 

[Fishrrman]

OP:

Try this. It won't take long and won't hurt anything.

Boot to INTERNET recovery (this is not the same as "the recovery partition"):
Command-OPTION-R
at boot

The internet utilities take a while to load, be patient as the globe spins.
If you connect via wifi, you'll need your wifi password.

When the utilities load, open disk utility.
IMPORTANT STEP: go to the view menu (upper left) and choose "show all devices".

Can you see the fusion drive?
Does disk utility offer any option to repair it?

It's becoming more and more common for iMacs that were sold with "fusion drives" to have problems with one of the drive components failing. Either the HDD portion or the SSD portion.

When this happens, the computer won't boot.
And... you'd better have a backup for your data, it is VERY difficult to get data from a failed fusion drive. Essentially impossible for most users at home if one of the drives has failed.

Having said this, you can still boot and run the computer from an EXTERNAL boot drive.

I'd suggest an external USB3 SSD.
Something like the Samsung t7 "shield" comes to mind.
It's very easy to set up an external boot SSD, and it may actually boot/run the Mac FASTER than the internal fusion drive did.