Comment by ChildOfChaos

i've got some issues with my older iMac from 2015 after my internal SSD failed and i'm booting from an external that all appear to be software related with little I can do about it as my Mac is out of support, even though these issues must have existed previously, it's so frustrating.

-- On Monterey no disk images will mount and no external drives or SD card. Everything is fine on older versions of OS, some forum posts suggest others with the same issue, but without any resolution. The only way to get DMG images to mount is to use a third party mounter like FastDMG, upgrading to Ventura fixes the issue with Disk images and my SD card, however this is not an officially supported OS for my model, so I have to use OpenCore Legacy patcher to achieve this. External drives still do not mount, the OS just doesn't see them at all, even in the recovery mode for these OS versions However running another MacOS inside Parallels I can access them, so they must be accessible somehow.

-- This is all perhaps caused by my firmware being out of date, i'm running 173.0.0.0 and it should be something like 530.0.0.0, this is updated with the operating system, however it has been found that on custom order macs with SDD's rather than the standard fusion drive, such as mine, the firmware update fails due to it incorrectly looking for a drive via the sata port. Apparently this is fixed on 195.x but you have to open up the iMac and plug in a drive into the Sata port just to get it to do this.

Extremely frustrating considering I paid a lot extra to spec this model up but due to Apple's mistake with the firmware updates (Or other unknown issue) I'm left with these problems. Since the machine is no longer in support, Apple aren't interested in helping.