el capitan vs high sierra is it worth the bother to upgrade

Well, if you like a supported system, High Sierra and its version

of Safari does see some security updates. El Capitan does not.

Depending on what your Mac could run, that may be a good bet.

However if El Capitan is end-of-road, that's too bad. If you try and

make the upgrade, be sure & have at least two separate backups*.

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

If you need to know 'how to create bootable installer for macOS' see instructions here:

• How to create a bootable installer for macOS - Apple Support

https://support.apple.com/en-us/HT201372

*One of them can be Time Machine, on external USB hard drive

& other, could be Clone. ~Or other duplicate; maybe disk image.

Be sure to have at least one, verify they're up to the task.

The iMac Late 2009 model has support verification to run

High Sierra 10.13.6. That could automatically change drive

formatting, from HFS+ to APFS; a newer later Apple format.

macOS Catalina 10.15 can upgrade Mojave, High Sierra, Sierra, El Capitan, Yosemite, Mavericks

There may be a prompt where it could 'ask' so, before doing.

Seems if Mac already has & supports an SSD - then APFS is used.

All the 'installers can be downloaded' as such. Direct from..

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

..links I posted (old systems, up to Big Sur. ~ OR, in link:

• How to create a bootable installer for macOS - Apple Support

Both are good references to understand ramifications.

You may end up with 2FA (two factor), but maybe not.

...I've downloaded High Sierra, but not installed it yet.

With two older Macs, Late 2012 i7 Mini & MB/Pro 13-inch

Mid 2012; these could run Catalina; but not yet.

Also you do have more compatible web browsers available to you.

https://discussions.apple.com/docs/DOC-250003586

And with 10.12.4, Two factor authentication allows you to see codes sent to your device when you login your Apple ID. This is more secure than the generic single factor authentication. Once updated to two factor authentication on your AppleID, you can't turn it off.

"Some Apple IDs created in iOS 10.3 or macOS 10.12.4 and later are protected with two-factor authentication by default. In this case, you see that two-factor authentication is already turned on." from the link you provided.

I presume the person is running El Capitan or lower or else the person wouldn't be asking about scenarios running it. The person already has an Apple ID or else wouldn't be posting here. In other words, yes, the person can upgrade to 2 factor authentication if they wish to and haven't already but Two Factor Authentication is not a requirement at least for running Mojave or lower (I know, I have run both on my system and I do not have 2FA) if you don't want to use services that require it. You'll just get constantly pestered about upgrading.

My AppleID had already changed to Two Factor, and my wife trying to connect to the AppleTV registered under my ancient AppleID did not work with Airplay until I upgraded it to Mojave from El Capitan.

Which is why I caution in the first place.

I'm running 10.10.5 Yosemite on an old iMac late 2009 the internal drive died long ago so am using 2 external G-Drives 2TB each one is for Time Machine the other is for the iMac which has 1.94 TB of available space.

I want to upgrade to El Capitan and or further since most app upgrades are for the higher OS X versions, El Cap is available from the AppStore and I'm guessing there's enough room on the ext G-Drive to do this, do you still recommend that I create a bootable installer on usb thumb drive?

I have run El Capitan, High Sierra, and Mojave all briefly on this Mac. None of them required me to use Two Factor Authentication (2FA). There may be some additional services requiring online access that any system version may offer as options that will now require 2FA but basic use of the operating system will not require this, at least as far as Mojave I can tell you. More services may become available as you move to higher systems and those services may require 2FA, but it isn't the system requiring it, just the fact that you now are now using a feature that that system introduces where Apple requires 2FA to use it.

When downloading upgrades & updates from Apple Servers

you might find Safari could be only browser to allows that.

Some of them may or not download; or may/not fail to work.

Since other users have better ideas, be sure to ask one.

Adios and happy trails!🙂

When you're referring to 2FA do you mean to log into Apple Support or just to access El Cap, High Sierra and Mojave?

I already encountered 2FA just trying to log into Apple with the number sent to my email, is that what you mean?

I'm guessing I have to use El Capitan to go further up the macOS chain correct?

I'm already surviving on an external G-Drive since the iMac internal fried itself long ago, plus another G-Drive for Time Machine.

Do I need to eject the Time Machine drive before/while installing El Capitan which is downloaded but haven't installed it yet?

Thanks for your help btw.

Two-factor authentication for Apple ID - https://support.apple.com/HT204915

Says you can use it with El Capitan.

Thank you, Safari has been a bit buggy so I use Chrome up to date, will that be an issue?