OSX can't be installed on this disk. MacOS isn't installed

The first option is a nonstarter for the simple reason that we cannot install an older version of OS X / macOS on a drive being used on a system running some later version.

Stated another way, the format of the external drive doesn't matter, nor does the format of the startup drive / volume. The limiting factor is that the Mac isn't running Yosemite, or something earlier.

We can't even create a "bootable USB installer" on a Mac running anything later than the installer you're trying to create. You must boot Yosemite, or an earlier operating system. Then you're in business.

Some time ago I successfully installed from the same installation package to a different Mac OS Extended (journaled) external drive.

Can't explain that one. Too many unknowns.

Workaround? If your MBP was even capable of running Yosemite, boot from its original, grey, System Install DVD assuming that Mac shipped with one. Install OS X on an external drive, and boot from it. Then, you're in business.

Didn't ship with DVDs? Boot OS X Recovery, which may convey the ability to reinstall that Mac's originally installed operating system. Same idea.

So to confirm the sequence, you downloaded the OS X 10.11 installer on your macOS 10.13 system, then used createinstallmedia to build an installer, and then booted and installed macOS from that installer?

I can only guarantee what I have done myself, and I am less than 100% certain it won't work, only because I haven't tried it recently — and recently, older macOS versions are becoming increasingly difficult to install.

I tried the Recovery route you suggested, but Recovery only allowed me to re-install High Sierra, with no option to access the version of Yosemite (the one giving me difficulties) attached on an external drive.

Boot Recovery using one of the key combinations that result in booting that Mac's originally installed operating system. Which one? Good question because recently (that word again) Apple added some language to the an earlier version of their Support document How to start up from macOS Recovery - Apple Support. It was confusing enough, and has since been superseded.

• Try each key combination: Command-R, Shift-Command-R, and Option-Shift-Command-R.

One of them resulted in booting the Mac's originally installed operating system. Which one I don't happen to recall. The previous version explained it better, but with "weasel words" added to it, saying one key combination "might" result in this, another "might" result in that.

Try each one.

In the distant past I made Yosemite bootable installer with this command but it might not work anymore if Apple has changed things (in later macOS the "--applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction" at the end of the command has not been necessary):

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app --nointeraction

Create a bootable installer for macOS - Apple Support

Or, boot to Internet Recovery via Shift-Option-Command-R which should let you install the earliest supported macOS, which might then be old enough to let you to upgrade to Yosemite.

John Howarth wrote:

I have now successfully used Recovery mode to install the original system from my Mac (Yosemite) onto an external drive. This successfully boots up as a new Mac, but so far I have been unable to access any data from my existing internal drive.

What format is the internal drive -- did you say in your 1st post that it is APFS? If yes, the there is your answer because Yosemite does not support APFS like High Sierra does.

Matti Haveri pointed out the fact Yosemite won't be able to read an APFS formatted volume so that's the answer to that question.

What you can do though is to boot your (now dual-boot) Mac into High Sierra which will be able to read the external HFS+ formatted volume. In that manner you can copy files from the internal to the external drive.

The major caveat is that any work products created by programs designed for High Sierra may not be readable by their earlier, Yosemite equivalents. Basic, generic files such as JPEG images, ASCII text files, certain word processing or spreadsheet documents, that sort of thing will be ok.

Which model year MacBook Pro?

Identify your MacBook Pro model - Apple Support

The usual format for older OS X versions is GUID partitioned (GPT) storage, with HFS+ case-insensitive.