macOS Catalina

create Mac OS Catalina bootable USB from UNSUPPORTED Mac OS version

I have two MacBook Pros

One supports Catalina (A1398) – but the drive died and I have replaced with a new, empty drive.

The OTHER one does NOT support Catalina, it's too old (A1278), it's running macOS High Sierra 10.13.6.

How ON EARTH can I use my functional (but outdated and unsupported) Mac to download and create a bootable USB for Catalina to rebuild my supported MBP?

I have gone round in silly circles with App Store, it seems "Apple knows best" and refuse to let me download the installer since I'm doing it on an unsupported Mac.

This is of course a flawed assumption since I have NO intention of installing it ON the Mac I'm trying to download from.

I have tried around a dozen times to perform Internet Recovery on the newer MBP with new drive – it always stops after I have hit "continue" on the install Catalina link after a minute or so with "could not contact a recovery server"

I have a very fast fibre connection, and there is no firewall on outbound comms, I am using WiFi but have no method of plugging this newer MBP in with a cable to eliminate WiFi use. No other devices have been conducting "high usage" activities while I'm trying to perform the internet recovery.

I am aware of unsupported methods such as "catalina patcher" that allow installing/upgrading on Macs that would otherwise fail Apple's compatibility checks. I'm not interested in running unvetted third-party tools like that – don't trust them.

Does anyone know how I can manually DOWNLOAD and create a bootable USB Catalina installer from my working High Sierra MBP?

I found a link that suggested terminal commands like:

softwareupdate –fetch-full-installer –full-installer-version 10.15.3

But that gives errors like:

softwareupdate: unrecognized option `–fetch-full-installer'

softwareupdate: unrecognized option `–full-installer-version'

usage: softwareupdate <cmd> [<args> …]

The "help" for that particular command doesn't help me understand and doesn't match what I read online, so I guess what I'm reading online caters ONLY for Catalina terminal app 🙁

This is a completely ******** decision by Apple not to provide any sensible options for downloading and creating offline media FOR different versions of MacOS to the one you're running!

The only viable method I have come up with is (I think) the following:

Use my working High Sierra MBP to create an otherwise pointless "High Sierra bootable USB"

( note : I tried to build one for Mojave but apparently that's not supported on my A1278 either…)

Use the High Sierra bootable USB to install on the new empty drive in my other MBP

Update from High Sierra to Catalina on the newer MBP.

Thanks Apple! 🙁

If anyone has a better idea for workflow using SANCTIONED methods and sources – I'm all ears…

Cheers,

theUt

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