Startup Disk Update Error Doesn't Go Away

[potassium507]

I have a 2018 Macbook Air that is running the latest version of Catalina, which I want to downgrade to Sierra (10.12.6). I followed these instructions: https://www.macworld.co.uk/how-to/mac-software/downgrade-macos-3581872/

Everything worked fine until it came time to actually boot from my bootable hard drive (2TB seagate portable drive). It has been erased correctly (MacOS extended journaled, GUID partition map) and successfully turned into a bootable according to the instructions in the link. When I view the disk in Startup Manager, the disk icon is orange and its name is "Install macOS Sierra macOS, 10.12.6"

I have disabled the T2 security, it is on the No Security setting, there is no firmware password required, and it is set to allow external drives.

When I boot while pressing cmd+R and choose the bootable disk, it tells me "A software update is required to use this startup disk." I press update, it reboots, then gives me the same message. This has happened over and over again. Once in a while, the message switches to "An error occurred while installing the update."

I have no idea what to make of this, and can't seem to find any answers anywhere. PLEASE HELP I feel like I am going insane.

 

[Fishrrman]

I don't know if this will work, but TRY IT anyway:
WARNING: this will wipe out any data on the drive.
DO NOT DO THIS unless you are backed up or don't care about erasing the drive!

Reboot to a special version of "internet recovery" using this key combo at boot:
shift option command R

Does it work for you? (it may, or it may not, the only way to know is to try it)

If it DOES, it will enable you to download and install the earliest complete version of the OS that originally shipped with the MBA you have.

IF you get a good boot, try this next:
1. Open disk utility
2. Check to see if there is a view menu. If there is, check it to see if you have the option "show all devices". If you do, choose this. If you don't see it, don't worry about it, just go to step 3.
3. Look on the left and click "the top line" which represents the physical drive inside the MBA.
4. Click on that and click the erase button. Choose "Mac OS extended with journaling enabled, GUID partition format"
5. Erase the drive. When done, quit disk utility.
6. Now open the OS installer and see if it will proceed.

Can you get this far?

 

[potassium507]

Okay so I am on step 4, and it shows me this. Is this normal?

 

[Fishrrman]

"Okay so I am on step 4, and it shows me this. Is this normal?"

I'm having a problem here.
Is it Low Sierra you want to install (OS 10.12)
or
Is it HIGH Sierra that you want to install (OS 10.13)
???
(I would verify which version of the OS it shipped with, but everymac.com seems to be down right now -- they would have that info).

For Low Sierra, I would erase it to Mac OS extended with journaling enabled (also called HFS+).
For High Sierra, try APFS instead.

If it was in my hands right now, I'd try HFS+ (Mac OS extended) anyway, and see how it goes.
If that didn't work, I'd start over and erase it to APFS, and see how THAT went.

At this point, it's up to you.

 

[CoastalOR]

I have a 2018 Macbook Air that is running the latest version of Catalina, which I want to downgrade to Sierra (10.12.6).

A 2018 Macbook Air does not support any macOS lower than Mojave. They originally shipped with 10.14.1 and Apple does not support any OS below the original shipped OS for that model.

 

[potassium507]

"Okay so I am on step 4, and it shows me this. Is this normal?"

I'm having a problem here.
Is it Low Sierra you want to install (OS 10.12)
or
Is it HIGH Sierra that you want to install (OS 10.13)
???
(I would verify which version of the OS it shipped with, but everymac.com seems to be down right now -- they would have that info).

For Low Sierra, I would erase it to Mac OS extended with journaling enabled (also called HFS+).
For High Sierra, try APFS instead.

If it was in my hands right now, I'd try HFS+ (Mac OS extended) anyway, and see how it goes.
If that didn't work, I'd start over and erase it to APFS, and see how THAT went.

At this point, it's up to you.

I want to downgrade to low sierra (10.12.6 specifically), this Macbook shipped with 10.14.1 installed.

According to CoastalOR it seems that it may not be possible to do the downgrade at all?

 

[chrfr]

I want to downgrade to low sierra (10.12.6 specifically), this Macbook shipped with 10.14.1 installed.

According to CoastalOR it seems that it may not be possible to do the downgrade at all?

That’s correct, that computer cannot use anything earlier than 10.14.1.

 

[Fishrrman]

If it shipped with 10.14.1, that's "the earliest OS you can use".

The only way to run Low Sierra on that one would be by using a "virtual machine" you create with Parallels, VMWare fusion, or the free "virtual box"...

 

[RogerWilco6502]

That’s correct, that computer cannot use anything earlier than 10.14.1.

I can second this. In general, Macs can't boot versions of the Mac OS lower than what they shipped with. For example, my MacBook5,2 shipped with 10.5.7. I put in a 10.5.0 disc just to see if I could get it to boot so I wouldn't have to install on a different machine and transfer it over (I figured it wouldn't work, I just wanted to try) and, as expected, it didn't work.