Re Update
Hi,
This is a reply to all those who have tried to help and as an aid for others who may want to download another MacOS to an external blank drive for trial or as an essential always available backup system drive.
In addition to the method stated previously I thought it might be possible to create a bootable drive that would be recognised by Startup in recovery mode but the support pages covered instructions only for system administrators needing to install the OS on a lot of machines, it meant more time on my part and using terminal instructions with no certainty of success.
I therefore contacted Apple support who were very helpful and advised that sometimes external drives are not recognised for Startup even though they appear to be normal in Disk Utility, this could be due to drive or formatting errors, cables or connections, or ports on the Mac; so these are the steps taken for my iMac ( Intel processor, no T2 chip, no Startup Security Utility, internal & external SSDs) using Internet Recovery:-
1. Check backups available on separate drive
2. Reset System Management Controller to discharge all residual energy from ports. See
https://support.apple.com/en-gb/HT201295 )
in addition click Start Button 5 times before reconnecting cables and restart.
3. Restart as normal and re-enable Finder in iCloud if necessary.
4. Shutdown and Restart into Internet Recovery manually, at Startup press and hold Shift-Option-Command-R
until the spinning globe appears then:-
a. Make sure the language selected matches your keyboard.
b. Check upper right to make sure you have an Internet connection.
c. Access Disk Utility, highlight the external drive you want to use and select erase. In the dropdown boxes enter
i. Name e.g. MyVolume, Big Sur etc.
ii. Format APFS (assuming its an SSD)
iii. Scheme GUID Partition Map
d. Execute and check it’s successful, in the Side Bar it should show the original Drive Name, a Container, and a Volume with the name you entered above with free space very nearly equal to the size of the SSD. If the drive also shows other volumes with same capacity it probably doesn’t matter these ghost volumes probably wont appear when using the system Disk Utility nor affect the procedure. For more information About MacOS Recovery on Intel-based Mac computers see https://support.apple.com/en-gb/HT201314
5. Close the Disk Utility and open Startup (top left menu), you should see and be able to select your new ssd volume
and close.
6. From the Utilities Window chose the MacOS you want, this should give you the option to confirm the download
destination.
7. The download will probably take 40 min or more.
8. On completion Shutdown and then Press and hold the Option key as you start up your Mac.
If you want to change the default Startup Disk you will need to change the drive selected in the Startup Utility in System preferences. See https://support.apple.com/en-gb/guide/mac-help/mchlp1034/11.0/mac/11.0 )
Thank you to everyone that responded
