Arrrgghhh... iMac disaster... need help!

[rexone]

Okay, short version.
Pulled our late 2014 BTO iMac (3Tb Fusion drive, 32G RAM) out of storage for our daughter to use.
Initially was just fine, updated OS etc. (not to Catalina but to reasonably recent Mojave)
Then it started to go flaky. Very flaky.
Slow to log-in & load apps or wouldn't log-in, would just get stuck and then it got worse & worse. 5-min to log-in, another 5 for finder & desktop to load, another 5 to open anything etc. Spinning beach balls for everything.
Ran diagnostics & no hardware issues reported. Repaired permissions, zapped PRAM etc. etc. but to no avail.
Thought I'd then try to do a clean install of Mojave. Made a bootable USB, that's all good, changed the start-up disc to that. Worked fine.
Wanted to erase the drive before installing so ran disc utility & then things got weird.
Wouldn't erase. It just hung with spinning beachball. Had to crash out of it because I left it for over 1-hour & was still spinning with no change in progress bar & no options responsive.
So I thought I'll just start back up & install without erase and now... the drive doesn't show up.
Obviously the erase has part-worked & now the drive is all but invisible so can't use disc utility on something it can't see...
Anyone have any suggestions for how I can access the drive to erase it & then run the clean install?
I do have a Time Machine back-up so shouldn't have lost anything.
Any assist appreciated.

 

[Rodan52]

Sounds like the HD has died. This can happen when a ageing HDD is subjected to rewriting. That is, the process of writing over the existing data causes "bad blocks" on the disk.
You can try booting from the Time Machine backup. Turn the device off, connect the Time Machine backup drive and boot holding the option key down until you hear the chimes or see the Apple logo.
What you should get is a window the same as the Recovery Partition window.
That will include Disk Utility. Select DU and attempt to use that to erase the HDD. If it fails you most likely need a new internal drive. If you go for an SSD you will likely get improved performance. If you decide to go that way you can instal a new OS using the same method as above.

 

[rexone]

Thanks Rodan.
Bit of an update...
Rebooted (again) from the USB & was able to see the drive.
Attached is a screen shot & I'm just running first aid in Disk Utilities.
This shows the drive isn't mounted. Should I try to mount the drive?

 

[Rodan52]

Thanks Rodan.
Bit of an update...
Rebooted (again) from the USB & was able to see the drive.
Attached is a screen shot & I'm just running first aid in Disk Utilities.
This shows the drive isn't mounted. Should I try to mount the drive?

Yes, you can try. I see now it is a fusion drive which complicates things a little.

 

[mj_]

Sounds to me like your hard drive is dying. Can you install any version of macOS on an external drive, boot your iMac off that external drive, and run a SMART utility?

 

[Rodan52]

If that doesnt work and I fear it wont I would try the first suggestion. Boot from your TM drive then use the Reinstall OS option. This will return the iMac to its original macOS. If all goes well you can instal Mojave from there.

 

[rexone]

If that doesnt work and I fear it wont I would try the first suggestion. Boot from your TM drive then use the Reinstall OS option. This will return the iMac to its original macOS. If all goes well you can instal Mojave from there.

Thanks. Funnily enough I tried something similar already & it wouldn't do that. It said that the version it was trying to install was older then the installed version so couldn't be installed...
That was when I decided to try & do a clean install.
I now seem to have had some measure of success.
I managed to run disc first aid & it failed but I then restarted & this time the drive had mounted so first aid must have at least partially worked.
So I then tried again to erase & it worked!
Installer for Mojave is running right now off the bootable-USB and seems to be running okay although it has stopped at about 80% on the progress bar & is say 'Estimating time remaining' but I'll leave it & see where it goes with that.

 

[Juicy Box]

The early 3TB Fusion Drives had a repair program due to a bad lot or something like that. I am sure that the repair program is over, and it may not have included the 2014 iMacs, but I think I have heard of many failing.

I had my Late 2012 1TB Fusion Drive slowly fail with similar symptoms to yours. I started noticing the symptoms about 1.5 years in, and it kept showing signs of failure over months and months.

It kept getting worse, like there was bad sectors on the drive and once I started using those sectors, the problems started, like slowdowns, slow and failed boots, restarts, and shutdowns.

I had Apple Care, but it didn't matter, because the Apple Store refused to replace the drive. Whenever they would test the Fusion Drive on their HW diagnostic tool, it would pass. So, they wouldn't replace it.

They would wipe the drive, reinstall the OS, and the problems would always show back up, leading to another Apple Store visit, which the drive would pass their HW test. Rinse and repeat.....

12 days before the expiration of the Apple Care warranty, the drive fully died. It did something to the boot loader when it died too, as it wouldn't boot into internet recovery nor the Apple Store's HW diagnostic software.

So, the Apple Store finally fixed a problem that should have been fixed a year earlier.

Anyone have any suggestions for how I can access the drive to erase it & then run the clean install?

I suggest replacing the drive, or running a boot drive externally. It is probably toast, and even if you can get it back, it may not stay back.

 

[Fishrrman]

As vertical mentioned in post 5, there were a rash of failures in the 3tb fusion drive model due to the 3tb Seagate HDDs failing. Most were in the 2013's, but perhaps this could be your problem with a 2014 as well.

My suggestion:

Fastest, easiest way to get the iMac running again would be to buy a USB3 EXTERNAL SSD, plug it in, and set it up to become the new boot drive. It's not going to be as large as the internal re storage space, but the computer will be running again. 500gb or 1tb SSD should do the job.

I would just leave the internal drives "in place, but unused".
- The 3tb HDD (which is probably the source of the problem) may not even spin up and run. If you can manage to "wake it up", I'd erase it and just "let it be").
- Once the fusion is "broken", you might be able to get the internal 128gb SSD running again. Actually, if it tests out ok, it could be a faster boot drive than an external SSD. But of course, space is limited and will have to be managed.

 

[smbu2000]

Have you tried to split your fusion drive? If the HDD is failing, then you may still be able to install macOS on the 128GB SSD blade of the iMac.

My late 2012 27" iMac has a 1TB Fusion drive and after deleting the FD and installing macOS I decided to recover from a TM backup that was bigger than 128GB and I remade the Fusion Drive to do it. Running macOS on the 128GB blade was fine though. You can use external storage (SSD or HDD) if you want more storage than that.

Unless you want to open up the iMac and physically replace the drive.

There is a section on the Apple website for rejoining a Fusion Drive (SSD+HDD), but there isn't an article about separating them.
You can follow the instructions here though:
https://www.ifixit.com/Answers/View/537462/How+do+I+split+Fusion+drive+on+Mojave+-+APFS

 

[rexone]

Thanks all, yes, taking this on-board but trying a step at a time.
Managed to get the OS installed & started loading everything from my back-up drive.
Problem now is that the transfer is running from a USB-3 drive at 10MB/s... then it dropped to 4MB/s.
That was last night so I cancelled the transfer, shut it all down & will try again this morning.
Just thinking from another angle, which this slow transfer speed has me doing, internally what are the USB ports connected to?
Could there be a cracked/damaged motherboard or some-such?

 

[Fishrrman]

"Problem now is that the transfer is running from a USB-3 drive at 10MB/s... then it dropped to 4MB/s."

My guess:
The internal 3tb drive is becoming marginal.
It probably won't get "better".

You'll eventually have to decide:
- open the iMac and replace the HDD
- leave the HDD "dead in place" and use an external SSD or HDD

Unless you're SURE of your abilities to open and repair the iMac without damage, or have access to a reasonably-priced repair shop, my recommendation stands at "door number two"...