Trying to do clean Catalina install: how big should the installer be?

You say once the Stub is launched it'll call and download the full version of Catalina. But then you say that that Stub cannot be used to create a bootable installer of Catalina. Eh? Perhaps what you meant was that instead of leaving the just-launched Install macOS Catalina on the screen and getting on with Terminal, I need to actually continue with what's on the screen, because it'll then contact those servers and download the full version?

And by 'Further, that stub cannot be used to create a bootable installer', do you mean that the stub alone can't do that; instead it also requires the full download? You can read your answers in two different ways, so I'm afraid you've left me rather confused.

My iMac's definitely compatible and indeed Catalina is now the newest macOS with which my iMac will operate, ie. for compatibility reasons, I can't go any newer than Catalina as an upgrade.

When I was trying out Catalina a couple of weeks ago, I used the bootable install from an ext drive method. That allowed me to boot into my normal Sierra or alternatively into Catalina. So why doesn't the full install of Catalina not work now? I was anticipating restarting the Mac with the Alt key down, that then leading to selection of the ext drive, then later being eventually presented with the Utilities window in which I could go into Disk Utility and erase my system drive, etc, etc.

Was support.apple.com/en-gb/HT211683 not the place from which to get the link for the downloading of macOS Catalina? If not, then from where? Downloads of these slightly older versions of macOS aren't it seems available on the Apps Store any longer.

I hope you can clarify this for me.

DI,

Re 'Boot your iMac from your bootable ext drive', I haven't got to that stage yet. I was in Terminal, putting in the command string that actually subsequently makes that drive bootable and into which Catalina gets put a bit later on. Just prior to that I'd downloaded the catalina installer again (having deleted the remnants of it from Applications and also having erased the ext drive afresh). But Terminal said it was unnecessary for the application path part of the command line to be included (Apple themselves say different). Terminal also said it couldn't find Install macOS Catalina, anyway.

HT211683 was indeed from where I did the download. It opens and launches on my iMac's screen automatically, but I needed to re-do the making bootable of the ext drive again, so I quit the launched installer, before using Terminal to insert the necessary installmedia command line. But maybe I should have left the installer open while doing that? Maybe that's why Terminal's refused to 'do the business'?

The download I used was on an Apple page (looked like an Apps Store page of old where macOS downloading's done), giving the full description of Catalina, and showing the size of Catalina (about 4.9GB or thereabouts, I seem to recall). But what downloaded was only a 19MB file.

My understanding was that, for a clean install, you must not proceed with the downloaded Install macOS Catalina that then initially sits on the screen semi-launched. If you do, it merely causes the existing internal system volume to be overwritten. That's NOT a clean install. A clean install is where you go through a process of erasing/ formatting the existing internal volume, before then installing Catalina into it. So it isn't until later that you should return to that semi-launched Install macOS Catalina. app and continue with it.

The small size of that installer file does, I admit, suggest that that file alone doesn't represent Catalina. But I'm struggling to understand why Terminal is objecting anyway. Several years ago I used the same methodology (of using an ext drive made bootable) to upgrade in a clean fashion from Mavericks to Sierra, and I don't remember having all this trouble.

DI,

Yes, you've slightly misunderstood what I wrote. I did indeed have a bootable drive a few weeks ago, but that was for trialing Catalina only. In preparation for permanently installing Catalina today, I erased that bootable drive completely, so at present it's not bootable and so needs to be made so by the familiar Terminal command string. As you'll appreciate, this then erases that drive once again, before transferring Catalina on to it. Then later, after I've erased the current internal Sierra drive, it'll be possible to transfer Catalina from the ext drive into the internal one. Thus a clean install.

I'm intrigued to find that you've managed to download Catalina from the Apps Store and that the download size is a straight 8.27GB. I've searched and searched the Apps Store but have failed to get it via that route . The only source I found of it was via a link in https://support.apple.com/en-gb/HT211683. But what gets downloaded is only a 'stub' installer apparently, which is a 19MB file.

It appears, from my searches, that other Mac users have been having similar problems when wanting to upgrade their existing macOS, to a clean install of Catalina, them like me getting just that 19MB file. Some Mac respondents elsewhere are saying that the full Catalina is actually no longer downloadable. I suspect, however, that what's obtainable may well depend on the macOS being currently used, and that's why you yourself got the proper 8GB download but I only got a 19MB download. By all accounts, though, my iMac's compatible with all the requirements for Catalina, and after all, two weeks ago I was running Catalina on my iMac in trial mode (using the ext drive). So, like you say, go figure.

I've now gone through and deleted all instances of Install macOS Catalina.app from my iMac, including from the Trash, and I'll try downloading it again, but I'm not all that hopeful.

Actually, how exactly did you yourself go about finding the Catalina download on the Apps Store? If the same method works for me, maybe I'll then find that full-download webpage.