2017 iMac - Permanently Disable Internal Fusion Drive?

[Raincoaster]

Hi everyone,

The Fusion Drive on my 2017 iMac finally died (SSD side before the HDD side) and I'm now running off an external SSD (which seems to be working very nicely, thanks to the help of @Fishrrman).

What I'd like to do now is permanently disable the Fusion Drive. What is happening is that on reboot, the Fusion Drive tries to boot itself up unless I hold down the option key and go to the drive select menu. Fishrrman provided some instructions on how to force the system to go to the drive select menu that he had come across, but I wondered if there were alternatives that would just see the drive completely ignored.

One option I thought would work is making sure that the external drive was selected in Settings -> General -> Startup Disk (see attached screenshot) but that doesn't seem to be honoured by the system. If left to its own devices, it will always try to boot from the Macintosh HD.

Any suggestions for things I can try that don't require performing surgery on the computer?

Thanks!

 

[esbardu]

Are you able to see the disks (SSD + HDD) using disk utility?

 

[Raincoaster]

Yes, I have something called "Container disk2" that lists a single drive 3.12 TB that says it's shared by five volumes. I see a system volume and a data volume, for what that is worth.

 

[bogdanw]

Unlike a PC, where you can disable a hard drive from BIOS/UEFI, you can’t do that on a Mac. You have to psychically disconnect it.
As a software solution, you could use a boot manager/bootloader (rEFInd/OpenCore) to exclude the internal drive from being loaded and always start from the external drive.

 

[Ben J.]

Unlike a PC, where you can disable a hard drive from BIOS/UEFI, you can’t do that on a Mac. You have to psychically disconnect it.

I have a slight warning: I once disconnected an internal drive on a 2010 27" imac, and it resulted in the fans running at full speed, apparently because if the temperature part of the connection was not connected to anything, it would confuse the OS. Just sayin'.

@OP:
I'd say just let the disk be, and stop shutting down your mac. Use sleep instead. And the few times you need to restart, you'll be reminded to press the option key, if you had forgotten.

 

[bogdanw]

I have a slight warning: I once disconnected an internal drive on a 2010 27" imac, and it resulted in the fans running at full speed, apparently because if the temperature part of the connection was not connected to anything, it would confuse the OS. Just sayin'.

As an internal SSD is cheaper than an external one, I imagine most people who go to the trouble of opening up an iMac would not only disconnect the drive, but install a new one.

 

[esbardu]

I understand that when booting from fusion drive you get an error. I am now using a 2017 iMac from external USB drive and in the internal fusion drive I have also a Mac OS installed. My iMac always boots from the external. You can try installing Mac OS in the HDD, leaving the SSD side unused and then maybe the iMac will be able to boot always from the external one.

 

[Raincoaster]

Unlike a PC, where you can disable a hard drive from BIOS/UEFI, you can’t do that on a Mac. You have to psychically disconnect it.
As a software solution, you could use a boot manager/bootloader (rEFInd/OpenCore) to exclude the internal drive from being loaded and always start from the external drive.

This is what I wondered - disabling a hard drive on a PCI is easy but there seemed to be no obvious way to do with with a Mac. The reality is I'm going to limp this thing along a while until hopefully, Apple comes out with something that works for me. I'd really like another 5K 27" iMac but that seems a pipe dream. I'll check out rEFInd and OpenCore.

Thank you!

 

[pshufd]

You could try formatting it without an OS on it. That's what I did on my 2015 but it was just a plain HDD, not a Fusion setup.

I have not seen a comprehensive document on dealing with broken Fusion drives and have just avoided them when shopping for old iMacs.