[SOLVED] Can't restore macOS from Time Machine after downgrading from Catalina to Mojave

[Kim Jong Greg]

Hi guys.

I cannot restore the files or the system from the Time Machine backup

Background info: I accidentally updated wifey's system to Catalina when doing some routine maintenance (too much time on my hands during quarantine, I guess). Quickly discovered the error of my ways when it transpired that her Adobe CS6 which she needs for work won't run (yeah, I found the Photoshop workaround but she specifically needs InDesign which is a 32bit app).


What I had done so far:

1. Verified Time Machine backup integrity
2. Made one final backup
3. Verified integrity again
4. Downloaded macOS Mojave installer from the APP Store (***technically went with a bootable High Sierra USB which I then updated to Mojave from another installer as trying to install mojave directly was throwing up some errors)
5. Created a bootable USB
6. Erased the main HDD
7. Performed a clean install of macOS and we're currently at macOS Mojave Ver 10.14.6

Problem:

1. During setup, I tried going for the Migration Assistant "restore from a Time Machine Backup"
2. On the "Transfer Information to This Mac" I can see the AirPort Time Capsule. Below there's the mac spinny thingy saying "Looking for other sources..."
3. I select the Time Capsule, click Continue and it prompts me to enter the password, which I do. (I tried entering it incorrectly on purpose to see if the input is accepted and it won't allow me to progress with the wrong password).
4. NOTHING HAPPENS - screen goes back to the state in point 2. I can once again select the TC, enter password etc.

For visual reference: https://support.apple.com/en-us/HT203981
I cannot progress past screen 4 on to screen 5

I went ahead and installed a clean OS, downloaded all updates so the only remaining one is the one to Catalina, which, obviously I will not be installing.


Additional possibly useful info:

1. on 3. I can hear the hard drive spinning up on the Time Capsule
2. when I'm in the OS I can see the Time Capsule on a list of my drives, I can connect to it in Finder and it shows a Data folder, inside of which there's one file: "wifey's iMac.backupbundle". It seems very large because in the time it took me to write this post Finder was still calculating size😉
3. I managed to establish that MacOS Catalina actually converted format of my Time Machine backups stored on my TM from the traditional .sparsebundle format to the newly introduced .backupbundle format.

Questions:

1. Do you think there is a way for me to somehow convert the .backupbundle into .sparesbundle?
2. Do you even think that is a possible correct solution?
3. Or should I focus on manually restore what I can? (To that effect, given it's the last existing copy of the entire 7 years worth of our data, can you recommend a safe approach?)

Lastly, wifey's System Specification : iMac 27-inch (Late 2013/Haswell) | macOS Mojave | Version 10.14.6 | CPU Intel Core i7-4771 3.5 GHz | RAM 32GB 1600Mhz DDR3 | GPU NVIDIA GeForce GTX 780M 4GB

Please send lawyers, guns, and money!

 

[Kim Jong Greg]

UPDATE -- SOLUTION (or lack thereof for the moment)

I had spoken with a senior Apple Support technician for two hours; we discussed the problem at length and tried several things that ultimately did not work. They had been very knowledgeable, clearly an enthusiast, and I don't think I would've had a different outcome having spoken to somebody else.

In short, this is a very fringe case, which was not envisioned as a problem scenario by Apple, as their current manuals and internal documents do not offer any solutions.

There is no known way to convert .backupbundle back to .sparesbundle that Apple supports, and both he and I had looked extensively for a third party solution which at the time of writing does not seem to exist.

This is not to say that a third party solution may not emerge in the future.

Aa such, a practical solution I am going to deploy is to upgrade back to Catalina, restore all my files and then run macOS Mojave in a VM, either with Parallels or Oracle (unless anyone is willing to recommend an alternative).