Problem re-installing Sonoma

[malizivko]

For some reason my iMac is extremely slow, and it has been slow for sometime now; sometimes it takes like 45 seconds to open up Safari.

iMac - Retina 5K, 27-inch, 2019
3.7 Ghz 6-Core Intel Core i5
Radeon Pro 580X 8GB
24GB 2667 Mhz DDR4

I have created bootable Sonoma and would like to completely re-install, but I am not sure what to do.

When I select drive where it should be installed it's telling me it can't be downgraded! Should I erase one of the volumes and which one?

Thanks

 

[rm5]

The fusion drive is likely the problem, and this has been brought up in the past. That's why your iMac is so slow. You'd need to go View > Show All Devices, select the topmost one, and erase it as APFS. Does that work?

In reality, I'd recommend grabbing an external SSD to install macOS onto.

 

[Bigwaff]

The one screenshot indicates you have 243.7 GB free of a 2.15 TB drive. Before you do anything, please back up your data... otherwise you will lose all your data if you follow @rm5 instructions to erase the device. @rm5 is correct regarding Fusion Drive. It's time to replace internal Fusion Drive or boot from external drive.

 

[CooperBox]

I agree with the above comments, ditch the fusion drive setup and install a 500GB or 1TB Crucial or Samsung SSD and give your iMac a new lease of life. Check out THIS link for the procedure. As the iMac is now 4 years old, if it were me performing this (labour free-of-charge) mod for a friend or neighbour, for convenience whilst the Mac was open I'd also re-paste the CPU & GPU. This together with the absence of the spinning HD would make for a far 'happier' cooler-running iMac.