Not able to upgrade from Mountain Lion to Catalina (or Mavericks) on my late 2012 mac mini with new SSD

You may try for El Capitan as that would be a newer old

system ~ up from Mt Lion & conditionally still available.

This may likely be key to go further up the chain; I believe.

• How to upgrade to OS X El Capitan - Apple Support

https://support.apple.com/en-us/HT206886

But if you use the links from various macOS upgrades you should be able to get one

much newer. Problem has been with those who have older 32-bit software and drivers;

while the latest macOS 10.15 solely uses 64-bit. That would be Catalina. Mojave is tamer.

My Late 2012 Mac mini 2.3GHz quad core (server) last model shipped with Mavericks;

that's still on HDD#1 on partition #1; while El Capitan is on same drive, partition #2.

This does OK, after I realized a need to boot into second system requires different

keyboard shortcuts. Now I seldom revisit Mavericks; it has near 500GB space on its

partition. Same for El Capitan's space. (My Mac mini has two 1-TB 5400-RPM HDDs.)

So the deal is mostly this; there are systems above Mt Lion that are internet

downloads that can be used. But your original Mac OS X is what will load by

default when the system is restored. (Unless upgraded further, and a firmware

update may keep it from using original Mac OS X. )

The other keyboard commands suggested in with later installer download

page w/ support articles with links to get them, can yield options; and you

can make the computer you have upgrade to later mac OS.

• How to upgrade to macOS High Sierra - Apple Support

https://support.apple.com/en-us/HT208969

I'd downloaded High Sierra installer about three years ago; it lives

as such in the Applications folder until used to get the actual 10.13.

system from Apple Server; or when I choose later one. There are

some issues some users had with various hardware; so these are

best not taken to the newest version. Each significantly changed

behind the curtain; the latest three, much more.

If your Mac mini has second hard drive, that could have originally been

a server; the HDDs are identical if from Apple. Otherwise there was a kit

with extra bracket (and perhaps second data cable for 2nd HDD.) There

are a few work-arounds if you choose to run with older mac system;

but that may be problematic.

Good third-party 'browser' and 'email client' applications are available

from mozilla (firefox and thunderbird.) Safari is only as up-to-date as

the system is, so it can be rather outdated. A few years behind, if not

given separate update steps along with security updates from Apple.

You should re-familiarize yourself with specifications & hardware

upgrades that your older Mac may need for a later macOS.

Good luck & happy trails!🌞🌤

The other item about El Capitan if used as everyday system, would be the Apple Mail

client no longer is up to date in several places and the TLS (what makes it stay current

in security protocols, etc to simplify matters) when not will not work correctly; so that

work-around would be to get and use Thunderbird email from Mozilla; this keeps you

the problem of outdated email. And also relieves you of managing email via Browser,

such as gmail login with separate browser windows, etc.

Because over many years of not using an email dedicated client software, I'm OK with

login for items such as gmail. But that has inconvenience and also the matter of google

tracking my email. I haven't gotten many ads or spam with gmail via webmail in Firefox.

The El Capitan 10.11.6 has been a fairly good Mac OS X; except being superseded by later

builds. And stable, almost as good as Snow Leopard, that was about the best of the former

era and also allowed older pre-X Mac applications under Rosetta. I still have an iBook G4

that shipped with Tiger 10.4(.11) and use TenFourFox browser. Does fair; slower on modern

web sites. This is also used for occasional webmail login.

If you had the hardware for it, El Capitan may be a good one to have on separate drive as

a clone. However its ability to be useful for security reasons may be coming to a close; yet

there are more than a few who still run older Macs. Firefox & Thunderbird are workarounds

and like anything 'tech' ~ they probably won't be updated infinitely.

Glad to have helped get past the initial setup issue from other to a newer macOS. I'd hold

off in the upscale wholesale adoption of Catalina for about six months; if you have older

hardware that needs 32-bit drivers and software that cannot go the extra marathon. There

are good sites and more info, this big transition been going on since High Sierra; and Mojave

is still in the running. My other 2012 Mac, a MacBook Pro, has Mojave; just not as an everyday

user. It came equipped with comparatively smaller 240GB SSD as an upgrade; I also upgraded

RAM to 16GB from 8GB.

Hopefully the rest of your experiments and updates go well.

Noted your question & forwarded request to another ASC contributor who

may be able to offer help; hopefully complexity of this issue has a solution.

With that said, I'm insufficiently experienced to offer one though..!

PSS:

The linked new thread you started above has seen several views & fair new attempts

to get more information for you; how close were they? A initial inquiry that directed

other ASC volunteers to look at your new thread, was within a 'closed' discussion.

So your input & participation is advised and input needed to see where this needs to go.

A direction for the volunteers to help look further, is advised. This peer-to-peer ASC often

is only other Apple or Mac product users who use their personal resources without any

compensation; so your interaction is helpful to guide & resolve the matter at hand.

Hopefully you are still with us ~ as far as your newer thread linked is to this new question.

{..An inside thread asked other ASC volunteers to look, too. ~ Several have answered via that..}

Best regards & wishes; and hope those newer questions, there, found you able to respond.