The following excerpts are from a previous thread Keyboard audio volume keys not working but, as that thread evolved into a whole different topic I’m starting one better titled for the evolved topic.
Okay….so now I’m going with the above advice. With the audio transfer tool recommended by artie, I have no need for the 21” iMac so I would like to clean it completely and have my daughter take it home for the grandchildren to use.

The machine is:

21” iMac, early 2008.
2.66 GHz Intel Core 2 Duo
4 GB of RAM

The goal: Installing the most current system the machine will handle, with the least fuss. I am just ordering a thumb drive, so no rush on the response, but I will need to know what to put on the thumb drive, where to find it, and how to load it onto the thumb drive...... and any other wise counsel.

According to Mactracker, your 20" Early 2008 iMac maxes out at OS X 10.11 El Cap.

You can d/l El Cap by simply clicking on the link on this page, although if the iMac is running OS X 10.7 Lion or later, I think you can do the entire job, including erasing its HDD/SSD in Recovery mode.

Hope that helps.

Before I had the issues described in the other thread (unable to get it past Snow Leopard 10.6.8) the machine had been running 10.11.6. Okay, good to know that’s where I max out.

Since that won’t be the process for me (currently 10.6.8), I assume I just poke the thumb drive, containing the El Capitan download, into a USB port and start the machine with the Option button pressed. Then I should be able to select the thumb drive and it will take care of erasing the drive and installing the system. Yes?

No... The El Cap d/l won't be bootable, so you'll have to plug in the thumb drive and double click on its icon to launch the installer, which will not erase the HDD/SSD.

You can make a bootable installer, but I'm not up on the details.

I'm not sure, though, that you can upgrade Snowy to El Cap without a pit stop in !0.7. Didn't you have a problem earlier? (More: Looks like you can.)

Personally, I'd really prefer to see the iMac's HDD/SSD wiped (in FireWire target disk mode), but unless you can get into Recovery, you'll need an adaptor (this one, I think) to do it.

More: I just took a look, and you may actually be able to upgrade to El Cap via Software Update. ... maybe

The instructions appear to be here.

Looks like it.

Please refresh my memory. Didn't you report in another thread that you were unable upgrade Snowy to El Cap on the iMac in question? And if so, I wonder if you'll be any more successful with the bootable installer?

Fingers crossed.

That's correct....for both El Capitan and Yosemite. In both cases, after the install process, the dialogue box said: "Installation Successful" and yet the system was still Snow Leopard.

In the recommendations to use a thumb drive, among the reasons from joemike, were:

As I said: Fingers crossed.

Almost there but still no cigar. I got through the entire process until the spot where I enter ‘y’ in Terminal to erase the thumb drive. Instead of erasure I got an error message: “Failed to start erase of disk due to error (-9999-0)”

I found a list of main causes for error messages and one of them was “Incompatible or outdated software/hardware installed on the machine.” Hmmmm. Could it be the thumb drive? I didn’t give it any thought when I purchased, so the drive is USB 3 whereas the old Mac would be USB 2

USB 3 shouldn't matter, because it's backwards compatible with USB 2, i.e. it'll work, but with USB 2 limitations.

As long as you're working with at least a 14 GB (as per Apple) thumb drive, I'm not aware of any reasons why you should be having problems. But then again, I've never been through the process and am not aware of its possible nuances.

It's 64GB. Maybe there's another way to make a bootable thumb drive.

Check out Install Disk Creator. I have used it in the distant past and it is easy and seamless. The linked page provided also lists other methods to create a bootable thumb drive.

Thanks for that....it looks like a helpful piece of software....if I can get to use it. I've run into a more basic issue: I am unable to get the material I need while on the 20" iMac. Safari will not link to the download sites, instead saying that it cannot make a secure connection. In fact, there are very few sites that Safari (V4.0.3) will connect to on that machine. ?? Thinking there might be a broken bit in Safari, I reinstalled Snow Leopard, but nada. I then rebuilt the directory with Disk Warrior, but nada.

I tried to make the bootable thumb drive on my 27" iMac, but I get a dialogue box that says that this version of the OS cannot be installed on this machine. Since it appears the bootable thumb drive can only be made on the machine it's to be used for, I am wondering if I should burn the OS and Disk Creator to a disk and use it to install them on the 20".

I have the original install discs for the machine but am not anxious to go back through that process again, as it didn't generate any positive results the first time I tried it.

1. Have you looked at Software Update on the 20" iMac to see if it offers you anything helpful?

2. Even if you d/l Install Disk Creator to your 27" iMac and transfer it to the 20" on a CD, I wonder if the available version will run in such an old version of the OS?

3. I've kinda lost track of what's been going on, so this may be a dumb question, but have you tried d/l'ing El Cap to the 20"er via the link on https://support.apple.com/en-us/HT201372 (How to create a bootable installer for macOS) and running the included installer, or have you just been trying regular stand-alone installers?

4. This is one of those instances in which I'd love to have the Mac under discussion on my desk in front of me!

Yes...have done that this morning and went through two updates so that Safari is now 5.1.10 and it connects to various sites without issue.

Yes, again....and, in fact, am currently in the midst of it. However, the download doesn't appear to want to stop. So far it's at 8.02 of 5.78 GB.....and still going.

Update: Gave up on the download and stopped it. I've cleared the Download list and have started afresh.

Well, I got the right download et cetera but, as previously, at the point where I enter ‘y’ in Terminal to erase the thumb drive, I get an error message: “Failed to start erase of disk due to error (-9999-0)”

But it doesn't offer you an OS upgrade?

Brain( )storm: In your other thread, you reported that you remain at Snowy even though you've run upgrade installers, and I'm wondering if SU is "mum" on upgrades because it thinks you're already up to date?

I've got a file, /Library/Receipts/InstallHistory.plist, in which a record of all installers I've run is maintained, and you should have an identical or equivalent file in Snowy. I wonder if SU will refresh itself if you open that file and delete every reference to the upgrades you've run, and then restart?

i dunno...worth a try.

Have you tried to simply run the included installer to see if it will upgrade you from Snowy (without erasing your drive)?

Yes....tried that twice in the other thread - once with Yosemite and once with El Capitan. Each time the dialogue box advised "Installation was successful" and each time there was no change. The OS remained as Snow Leopard.

I knew you had tried, but I wasn't sure which incarnation of the installer you tried with. I was wondering if you tried with the version you d/l'ed to create the bootable installer...not that I really think any particular installer will work differently than others, but because at this point, anything you can try is worth a try.

I'm not aware of any reason why Software Update isn't offering you an upgrade other than because it thinks you're running El Cap already, so I think trying the fix I suggested is certainly worth the effort. (Be certain to erase EVERY reference to any version of the OS beyond Snowy.)

Out of curiosity, what does /System/Library/CoreServices/SystemVersion tell you?

And if you'd like to cut straight to the chase, here's what may be your ultimate solution:

1. Erase your OTG Pro.
2. Clone Snowy from the iMac to the OTG.
3. Boot the iMac from the clone.
4. Erase the iMac with Disk Utility.
5. Install El Cap on the erased disk.

And if that doesn't work, Lt. Columbo is out of pesty ideas.
But I still suggest trying to tweak Software Update.

Or if you prefer, you can install El Cap on the erased OTG rather than cloning Snowy.

Given I had reinstalled Snow Leopard, and it went through a couple of updates, I thought I'd once more try upgrading....so I ran the previously downloaded Yosemite installer. After about forty minutes I got a mountain on the screen and an "About this Mac" that says Yosemite 10.10.5. Curiously, I tried to get updates from the web (as I did with Snow Leopard) but the machine said: "Cannot connect to the App Store".

The end goal is to give the machine to my grandchildren so I still need to get to a bootable thumb drive with El Capitan and, figuring the machine should be as up to date as possible, I am now running the El Capitan installer.

Gigantic Irony: This all started in the previous thread because the audio function keys (F10, F11, F12) were not working. After the current Yosemite install, they were still not working so, after a short bit of head-scratching, I swapped keyboards with the 27" keyboard. You guessed it....the function keys worked. Aaaargh!!

I think you can now forget about the bootable installer.

Since you're running Yosemite, you can boot into Recovery, where you'll have access to Disk Utility to erase your HDD/SSD, and to the various install options, one of which will be the most up to date OS your iMac can run, i.e. El Cap.

https://support.apple.com/en-us/HT204904 How to reinstall macOS - Apple Support

WOW! What an ordeal, particularly when you factor in the irony.

The link given to the December article doesn't help. The article pops up but the hotlinks embedded do not allow for downloading, irrespective of what the article says -- ie, it seems to be a dead-end.
Any other thoughts? I'm looking for downloadable High Sierra (10.13).

I'm not sure which article is "the December" one, but you should be able to d/l High Sierra via macOS High Sierra on the App Store.

More: I found that article, and the HS link takes me to the same App Store page as above. <How to download an old macOS installer | AppleInsider>
Dunno what's up on your end.

You cited <https://appleinsider.com/articles/21/12/03/how-to-download-an-old-macos-installer>, which stated one could download High Sierra from the Apple Store, but trying to access same gave no indication how that might be accomplished and the Apple Store page which showed up had no links to the appropriate site at the Apple Store with a link to download High Sierra 10.13.6.
Keep in mind that I have to be able to download the site/app via a non-Apple/Mac system in order to manage such download in finite time (ie, via a WiFi connection), given its 4.8GB size.
So, any other thoughts? Solutions? Resolutions?

When I click on your link I see this when I scroll down the page.
When I click on the High Sierra link I am taken to this Mac App Store "Preview" page (with its "View in Mac App Store" link), which almost immediately redirects me to this page in the App Store with that little "Get" in the top right corner.
If you're not seeing the same thing, I've got no idea what to tell you.

RE "When I click on the High Sierra link I am taken to this Mac App Store "Preview" page (with its "View in Mac App Store" link), which almost immediately redirects me to this page in the App Store with that little "Get" in the top right corner. If you're not seeing the same thing, I've got no idea what to tell you."

Apparently that's the problem. When I access the same page (from a non-Mac) the box with "View in Mac App Store" does NOT appear and is therefore not accessible.
If you've got a URL for the last page you've shown (with the "GET" box), please supply same. Otherwise I guess I'm SOL.

From all I can determine it is IMPOSSIBLE to find and download apps from the App Store unless one accesses the App Store via a Mac. [Even when I access my Apple Store account via a non-Mac, there's no way I can access apps, in particular the one I'm now looking for.] So, I guess I am definitely SOL.

That last page is an App Store page. It hasn't got a URL.

As you present the circumstances, I can offer you two options;

1. I've got an app that is supposed to enable me to send files as big as the HS installer via email, but to take advantage of it, you'll have to give me an email address and be able to do the d/l via a much faster internet connection than your own.
2. If you've got a street address that you feel free to post or email to me, I'll be happy to burn the HS installer to a DVD and snail mail it to you.

Alternatively, I suggest that you call Apple and see if they've maybe come to their senses and offer a means for you to get HS directly from them. Yours is the situation I railed about when Apple switched to its new d/l procedure, and although you're the only FTM poster to have run into the issue, I'm sure there are many others who haven't reached us, and perhaps Apple has come 'round to accommodating you-all.

Many thanks for the offer(s).
If I decide to upgrade to High Sierra, I can always have one of my tech savvy colleagues in my neck of the woods provide me with same.
Movin' on.

And, it ain't over. Couple of odd things with the machine (El Capitan 10.11.6):

Safari - When I go to many sites, including FTM, I cannot enter because Safari puts up a caution "This connection is not private". It has two buttons "Show Details" and "Go Back" but neither works. i.e. clicking on them doesn't cause any action.

Clock - It is about 2 1/2 minutes fast but, when I go into clock preferences, I cannot make any adjustments. The up/down arrows are greyed out. Update: Fixed the clock adjustment issue (took it off Set Date Time Automatically) although I wonder why the automatic setting couldn't get the time right.

Have you done an erase and clean install, which is now possible since you're in El Cap, on this Mac?

Considering all the issues you've suffered through to get to this point, I'm not at all surprised that an upgrade of the installation that spawned them is wonky.