Looking to flash older firmware onto an iMac 5k Late 2015 (17,1)

[KayvK]

I've extracted the firmware files from the Sierra installer. An older guide (https://forums.macrumors.com/thread...ld-efi-after-downgrade-from-yosemite.1803934/) said to flash the .scap files using the bless command. The only problem? Any iMac newer than 15,1 only has a .fd file. I haven't seen this before, and as such, I don't really know what to do with it, or how to flash it. I ran the command with the .fd file anyway, and it didn't reject it, but upon restarting I was on the same firmware.

Granted, the fact it didn't work could be caused by the replacement SSD, as I've heard BootROM / EFI updates being blocked because of replacement drives. But I don't plan to open the iMac and swap it out for the OEM drive, because the OEM drive is dead.

Here are the files:

Any solutions?

 

[profcutter]

My understanding is that the firmware which came with high sierra is what enabled the use of 3rd party SSDs. If you got the old firmware to install, your machine wouldn’t be able to see the internal drive you have installed. Why do you want an older firmware?

 

[KayvK]

My understanding is that the firmware which came with high sierra is what enabled the use of 3rd party SSDs.

I didn't realize this. I'll try with the High Sierra Firmware rather.

 

[KayvK]

Yeah it didn't do anything. Just asks for password, says nothing, and on reboot its the exact same.