Can the native Apple SSD of a iMac Fusion disk be ignored?

[komatsu]

I have a 2015 27" iMac here with a piddly 24GB SSD in.

I have "fused it" with a new SSD (Sandisk 1TB) but Catalina or Big Sur will still not install on it.

I have a strong suspicious the fusion disk is the root cause of the problem and I have no problem forfeiting that 24GB of space.

However, here is my concern:

On previous 2 previous iMac systems using Fusion drives - I ignored the native SSD disk and just installed the OS on the new SSD. On BOTH of these systems, within a week, the systems would no longer boot. They just went to black screen. The solution involved "refusing" the disks. Then they worked perfectly. Were these two cases just an anomaly? Can the native SSD on an iMac system be safely ignored?

 

[mpe]

Not sure why your previous Macs stopped booting, but I had Macs booting from different disks without problems. I guess it could happen when you reset NVRAM which forgets the startup disk preference, but in such case, you can always "fix" it by holding the "Option" key at boot and pick it again.

Also, the 24GB SSD is removable, so you can take it out if it bothers you.

 

[posguy99]

I have a 2015 27" iMac here with a piddly 24GB SSD in.

I have "fused it" with a new SSD (Sandisk 1TB) but Catalina or Big Sur will still not install on it.

If you took out the rotating rust and replaced it with an SSD, you should not have re-created the Fusion drive. What would be the point?

 

[komatsu]

If you took out the rotating rust and replaced it with an SSD, you should not have re-created the Fusion drive. What would be the point?

As I've said, on previous 2 previous iMac systems using Fusion drives - I ignored the native SSD disk and just installed the OS on the new SSD. On BOTH of these systems, within a week, both systems would no longer boot. It was if they system was looking for the native SSD - could not find it - and just gave me a black screen.

 

[komatsu]

Also, the 24GB SSD is removable, so you can take it out if it bothers you.

It that easy to do or does it require removal of the logicboard?

 

[mpe]

I think you need to remove the board (as it is on the other side). However, for me, the biggest hassle has always been removing and re-attaching the display. Once you are in, getting to anything isn't that big deal.

 

[posguy99]

As I've said, on previous 2 previous iMac systems using Fusion drives - I ignored the native SSD disk and just installed the OS on the new SSD. On BOTH of these systems, within a week, both systems would no longer boot. It was if they system was looking for the native SSD - could not find it - and just gave me a black screen.

Correlation is not causation, but hey, whatever.

 

[Fishrrman]

You could try this:

1. UNfuse the drives -- convert them both into "standalone", individual drives.

2. Erase the Apple SSD, and just "let it be" (let it sit there, empty)

3. Erase the Samsung SSD.

4. Try the OS install on the Samsung drive ONLY.

Will it install and boot that way?

 

[komatsu]

Ok, here is an update.

I went to Terminal and ran a

diskutil resetFusion

This seemed to execute but in Disk Utility the two disks were still fused. I did a reboot and the two disks were still showing fused.

So, I used a bootable USB installer and installed Big Sur. It installed perfectly this time. Ironically, the reset fusion command seems to have fixed the issue.

 

[mlody]

Ok, here is an update.

I went to Terminal and ran a

diskutil resetFusion

This seemed to execute but in Disk Utility the two disks were still fused. I did a reboot and the two disks were still showing fused.

So, I used a bootable USB installer and installed Big Sur. It installed perfectly this time. Ironically, the reset fusion command seems to have fixed the issue.

So are you using the Fusion setup with the built in 24 GB Flash Storage and 1 TB SSD, correct? I am planning to do the same, but my iMac is 2 TB Fusion so my Flash Storage is 128 GB.

I know many would say to ignore the 128 GB Flash storage and only use newly installed SSD, but I actually want to use it as I am assuming it would boost performance of the SSD, no?

 

[posguy99]

So are you using the Fusion setup with the built in 24 GB Flash Storage and 1 TB SSD, correct? I am planning to do the same, but my iMac is 2 TB Fusion so my Flash Storage is 128 GB.

I know many would say to ignore the 128 GB Flash storage and only use newly installed SSD, but I actually want to use it as I am assuming it would boost performance of the SSD, no?

No, there is no such configuration. Fusion drives are a small SSD paired with a large rotating rust hard disk. There would be no reason to back one SSD with another.

 

[mlody]

No, there is no such configuration. Fusion drives are a small SSD paired with a large rotating rust hard disk. There would be no reason to back one SSD with another.

Fusion in 2015 iMac is based on PCIe NVMe which is 4-5 times faster than typical SATA SSD, so in theory, pairing the Flash storage with SSD should provide performance boost.