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Help downgrading Monterey -> Sierra

Yes, I did the top level, even if I used the wrong word. I'll attach a screenshot.

I also tried - while booted from High Sierra - to do a restore onto the SSD from the Low Sierra boot drive - I got the following error: "Source volume is read-write and cannot be unmounted, so it can't be block copied... The operation couldn't be completed.....error 19" - is there perhaps a way to unmount it for such an operation?
 

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seinfeldseinfeld said:
Yes, I did the top level, even if I used the wrong word. I'll attach a screenshot.

I also tried - while booted from High Sierra - to do a restore onto the SSD from the Low Sierra boot drive - I got the following error: "Source volume is read-write and cannot be unmounted, so it can't be block copied... The operation couldn't be completed.....error 19" - is there perhaps a way to unmount it for such an operation?
Click to expand...
The problem you have is that your drive is simply not compatible with Sierra. Your computer needs to be running 10.13.1 or later to recognize the disk; drivers for it don't exist in Sierra. See here: https://www.owcdigital.com/assets/dealer/slicks/Product_Spec_Sheets/owc-aura-pro-x.pdf
 
Could the OP get a small external USB3 drive (such as a 128gb 2.5" form factor SSD in a USB3 enclosure), and then install Low Sierra onto it and boot and run that way?

After doing so, the internal 3rd-party drive may still remain "invisible", but at least he can be booted and running in Low Sierra.

The OP has never explained WHAT software he has that won't run in newer versions of the OS...
 
I APPRECIATE very much everyone's help on this. I even tried a clean install of Sierra via Target Disk Mode - it actually appeared to install and even booted ... at first ... but then resulted in the empty-set error. I guess I will have to swap out the SSD.

[[ I run Office 2011 which is slightly compatible with High Sierra but buggy when I tested it and not at all compatible with Mojave. If you ever saw Office 2011 on Sierra you would know why I absolutely don't want to "upgrade" - I spend a lot of time with those apps and 2011 is far better than the upgrades. I also use several other apps regularly that have no upgrades available (they've been officially discontinued by their makers, unfortunately), and I've searched for good Mojave alternatives, haven't found. Two utilize foreign languages and one is a teacher's gradebook app - that one I could probably find a replacement for, but then all of my old gradebooks would become inaccessible. ]]
 
I continue to use Office 2011 with Mojave, as it works well. Not sure what you mean by buggy with High Sierra, and it certainly works for everything that I need Office to do in Mojave. Office 2011 is one of the reasons that I continue on with my main home system with Mojave. I use both Word and Powerpoint on a daily basis. I never use Excel, maybe that is the difference for you.

That "prohibited" symbol means that there is something wrong with the system software, or the system is just not compatible with the Mac that you are using. You should be good with Sierra on a 2015 MacBook, so I would go with the idea that there is something wrong with the Sierra installer that you are using.

Where did you get that Sierra installer on a DVD?
Did you make that DVD yourself?
 
DeltaMac said:
I continue to use Office 2011 with Mojave, as it works well. Not sure what you mean by buggy with High Sierra, and it certainly works for everything that I need Office to do in Mojave. Office 2011 is one of the reasons that I continue on with my main home system with Mojave. I use both Word and Powerpoint on a daily basis. I never use Excel, maybe that is the difference for you.

That "prohibited" symbol means that there is something wrong with the system software, or the system is just not compatible with the Mac that you are using. You should be good with Sierra on a 2015 MacBook, so I would go with the idea that there is something wrong with the Sierra installer that you are using.

Where did you get that Sierra installer on a DVD?
Did you make that DVD yourself?
Click to expand...
Again, the drive isn’t compatible with Sierra because there are no NVMe drivers in Sierra. There’s no Sierra installer that’ll work with that drive and allow the computer to boot, so it’s not a matter of the installer itself having a problem. When the computer is in target disk mode, this isn’t an issue, and the computer running the Sierra installer doesn’t know that the destination drive is an NVMe drive so the installer appears to work.
The computer’s firmware has been updated to recognize that the computer has a drive in it, which is why it’ll boot later OSes, but when the boot loader tries to find a Sierra driver for the NVMe disk, the boot fails.
 
chrfr said:
Again, the drive isn’t compatible with Sierra because there are no NVMe drivers in Sierra. There’s no Sierra installer that’ll work with that drive and allow the computer to boot, so it’s not a matter of the installer itself having a problem. When the computer is in target disk mode, this isn’t an issue, and the computer running the Sierra installer doesn’t know that the destination drive is an NVMe drive so the installer appears to work.
The computer’s firmware has been updated to recognize that the computer has a drive in it, which is why it’ll boot later OSes, but when the boot loader tries to find a Sierra driver for the NVMe disk, the boot fails.
Click to expand...
One of these days, I will remember what you have posted here. Of course, my answer was incomplete. I will try to move forward with the information that the NVMe device, and Sierra, just don't talk to each other properly.
And, I think that OP's possible way forward with older software, is to install Sierra on an external drive, and boot to THAT drive when needed.
 
DeltaMac said:
And, I think that OP's possible way forward with older software, is to install Sierra on an external drive, and boot to THAT drive when needed.
Click to expand...
Yes, that or a virtual machine, but the external drive is probably the simplest option.
 
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