Success!!!
I finally managed to clone and create a MacOS 10.15 volume in addition to the 10.14 volume on the same boot drive. The solution to my Superduper problems was to boot from an external USB drive with MacOS 10.14 and Superduper on it. I also tried booting into safe mode with the internal 10.14 volume, but that didn't help either.
10.15 appears to work fine. If not I can always reboot into the 10.14 volume and delete the new 10.15 volume. Thanks to
@FreakinEurekan for the suggestion to clone the boot drive!
I also used
Retroactive to install iTunes on the new drive volume (after having upgraded it to 10.15) which worked perfectly. I also made sure to remove the new multimedia apps (
Podcasts,
TV and
Music as far as I remember) form the dock. I haven't yet tried running any of those new apps to see if they'll mess up the file structure, but I think I'll create a new and simple iTunes library to experiment with this (even though I've made sure to back up my existing iTunes library.
Here's a quick step by step guide on how I created two drive volumes so I can choose between two different OS versions (10.14 and 10.15 in this case), which is useful in case you plan to upgrade but aren't entirely sure if it'll run all your apps or otherwise work as intended:
1) Using
Disk Utility,
create a new APFS volume to the existing drive where you have MacOS 10.14 (provided there's enough room of course and you've made a recent backup).
Give the volume a descriptive name so as to distinguish it from the original 10.14 volume (for a while we're going to have two MacOS 10.14 volumes, so this will avoid any confusion to what is the original and what is the clone volume).
Read in detail how to add a new volume here:
Use more than one version of MacOS on a Mac
2) Get hold of the MacOS 10.14 Mojave installer.
I'm not sure of the current download locations as they appear to change all the time (that's why it's a good idea to download and keep them on a backup drive for later).
Also,
get hold of the MacOS 10.15 Catalina installer which we'll use later to update the 10.14 clone.
3) Use the 10.14 installer to
install MacOS 10.14 Mojave on to an external USB drive
4) When done, go to
"System preferences" - "Startup disk" and
select the external USB drive as the startup drive, then press "Restart" to
reboot
5)
Download the free version of Superduper to the external USB drive.
Install it in the usual app folder (/Applications/) on that same
external USB drive
6) Start the Superduper app (on the external USB drive).
Select the
Source disk volume (to copy FROM), the
destination (empty volume, to write TO).
And "
Backup -all files" as the copy method (which essentially clones the drive volume).
Press "
Copy now" to start the copy process
7) When done, Superduper should report a successful copy
8) We can now reboot into the new (cloned) 10.14 volume.
Go to
"System preferences" - "Startup disk" and select
the new MacOS 10.14 volume (which should be distinguishable from the original drive volume because of the unique name you gave it earlier).
Press "Restart" to
reboot
9) once having booted into the
new MacOS 10.14 volume it's time to upgrade it to MacOS 10.15.
Run the MacOS 10.15 installer and
select the new drive volume to be upgraded
10) We can now choose between using MacOS 10.14 or MacOS 10.15 to boot into, so as to "test drive" 10.15 and see if it works as intended, all while accessing your usual files and app.
Use
"System preferences" - "Startup disk" to reboot and change between the two operating systems.
11) Having tested out the new OS for a while you'll be able to decide which one you want to keep.
At this stage you can reboot into the drive volume you prefer, then run
Disk Utility and finally delete the other drive volume which you no longer want to use.
Or you can keep both drive volumes and be able to switch between two operating systems.
UPDATE: I also have some good news regarding iTunes (installed via
Retroactive as suggested earlier by
@conmee ).
After having installed iTunes on the 10.15 system I created a simple test library with a small variety of file types. I then quit iTunes and ran the various new media apps (Podcast, Music and TV).
It turns out they don't touch the iTunes setup at all, and files added to either of the new media apps don't appear within iTunes and vice versa.
I'm guessing the new media apps check to see if iTunes is present, and if it isn't (which is the default case when upgrading from an earlier OS -as iTunes is removed), the iTunes library is then reorganized. Just an estimated guess though, but so far looking good!
UPDATE 2: the above ("update 1") was actually tried out on my Mac Pro with a separate MacOS 10.15 SSD I booted into. On the Mac Mini however things were a little different in regards to iTunes.
Here I noticed that the media files and playlists from iTunes were available from the other apps as well (most notaly TV and Music. I think the Podcast app works a little differently since there's an option there for syncing via your Apple ID across your devices).
I checked the file locations for a couple of music files (by right-clicking and selecting "Song info", then "File") and saw that they were still within the iTunes folder.
Finally I did a little experiment where I create a new playlist to each of the apps (
iTunes -in the music section, and in the
Music app), then added (by dragging from the Finder desktop) a new MP3 to the new iTunes folder, and similarly to the Music app, but a different MP3 file so as to distinguish them.
Interestingly the respective files stayed in the apps they were added too (regardless of quitting and restarting the apps. I haven't yet tried to see if rebooting does anything differently though), but by checking their file locations (right-clicking the files and selecting "song info", then "File") I saw that in both apps, the new files were located within the iTunes folder.
Mind you, I haven't done any extensive research into this, but so far it seems running the new media apps (TV, Music and Podcast) appears not to mess up anything in regards to iTunes, so that's good news!
