Is it still possible to replace the logic board in my 2012 12 core Mac Pro? If so, WHERE can I have it done?

<< Thanks for the info on the Base System - I won't use it. >>

Au contraire, you HAVE to use it. The ONLY way to install MacOS is by using MacOS Base System -- as the system you are temporarily running, just not the install destination.

don't wait for anything -- get to work with the tools at hand!

You CAN'T delete MacOS Base System, and a disc does not contain ONLY MacOS Base System.

It is not on any ordinary drive. it is mounted as a RAM disk when needed.

What is supposed to be on those DVDs is a Bootable Installer. That means when it starts up, it will likely say Recovery, or possibly just Installer.

If all that was offered as a place to install was MacOS Base System, you did not properly initialize your drive yet. Choose Disk Utility off the Utilities menu in the Installer. You may have to back up, or possibly even reboot to get the Utilities menu to show. answer only the 'What language' question, and wait a minute for the Utilites menu to show up.

Choose Disk Utility.Select the proposed Install destination drive, by its immutable manufacturer-given name, and give it a name you like or Macintosh HD, and choose ERASE. You want GUID partition Map and MacOS Extended (journaled) format.

FULL MacOS does NOT run off a DVD. A DVD contains ONLY Installer and Utilities (powered by MacOS Base System) which you can use to Install MacOS onto a drive you have prepared.

You need to have a drive ready to accept a new copy of MacOS, which any of those discs SHOULD be able to Install when you boot from one of them.

Full MacOS requires about 9GB of additional drive space (not counting about 15GB of stuff installed) just to start up. That is way bigger than a DVD.

<< I have put both Mac OS 10.10.5 and 10.11 on my (previously erased) Samsung 2TB drive. >>

so do you have a running MacOS now?

You are supposed to hold the Option key at startup while you BOOT from one of those DVDs, and when you do, they are supposed to come up with an Installer screen and allow you to install.

As I said earlier once you can get ANY version of MacOS running off a drive, you can get access to the rest in a few more steps.

mgraffeo wrote:

I have put both Mac OS 10.10.5 and 10.11 on my (previously erased) Samsung 2TB drive.

What exactly did you put on the SSD? The downloaded DMG file? That won't help you at all unless you can have a working host Mac generally from 2007-2015 where you can run the included .pkg file to extract the real installer app into the Applications folder of the host Mac.

Here is an Apple article for downloading various versions of macOS installer including multiple steps needed for extracting the real macOS 10.10 & 10.11 installers from the .pkg/DMG into the Applications folder so that you can use the Terminal commands to actually make the bootable USB installer.

How to download and install macOS - Apple Support

Create a bootable installer for macOS - Apple Support

Also, my 2016 MacBook Pro says that it cannot download older versions of OS and stopped the download! Now, I will try my 2009 Mac Pro!

The 2016 MBPro should be able to do download both the macOS 10.13 and 10.14 installers.

However, you cannot run the installer app after the download completes if you have a later version of macOS installed. And running the installer app is not necessary since you only need to use the Terminal commands once the full installer app has been downloaded into the Applications folder.

Make sure the size of the downloaded installer found within the Applications folder is about 6.x GB in size. If it is only about 25MB in size, then it is only a stub installer. Let us know if this is the case since we will need to offer you instructions for using the command line to download the full 10.14.x installer.

Edit: Forget my reference to macOS 10.15 Catalina since a Mac Pro 2012 can only run up to macOS 10.14 Mojave. However everything else still applies. I've edited my response accordingly.

Edit 2: Changed reference of 2007-2025 to 2007-2015 since there is no way a 2025 Mac can run 10.11.

mgraffeo wrote:

I tried to download Mac High Sierra, but my 2009 Mac Pro - the only other Intel computer I have - is having its usual problem of not letting Safari connect to the Internet or Bluetooth -

Thus, I couldn't download any of the older Mac OS from the 2009. And so...I inserted the DVD of 10.11 into the top optical drive of the 2012, while holding down the Option key - after changing keyboards to a wired keyboard! - and it seems to be loading, as the progress bar is continuing its progress. I don't know if it will completely load the OS, as this disk could not load completely on the 2009 Mac Pro, but it is still going.

You can download the macOS 10.11 installer using any computer or OS that can access the Internet. Then transfer the downloaded DMG file to the Mac Pro where you need to run the .pkg file from the mounted DMG in order to extract the real installer into the Applications folder on the Mac Pro. Then run the real El Capitan installer located in the Applications folder to upgrade to El Capitan.

Once you have El Capitan installed, then you should be able to use that Mac Pro to download the High Sierra installer or Mojave installer.

mgraffeo wrote:

I am purchasing a USB with 14 Mac OS operating systems on it. My 2012 was running High Sierra, which is on this USB. Should I wait for it to arrive (from Ebay)?

Careful there. Although Apple once sold USB drives containing Lion, and USB drives containing Mountain Lion, Apple never sold USB drives containing installers for 14 versions of macOS.

That means that whoever is selling this USB drive is selling unauthorized copies of Mac OS X / macOS, and that there might be some risk of these copies having a few unadvertised "features" (i.e., bundled malware).

A safe place to get many old copies of Mac OS X / macOS is

How to download and install macOS - Apple Support

although you will need a working copy of the operating system to use these installers, and may need the help of another Mac or PC to download them if your current operating system is too old to access modern https sites.