Ventura install - major fail… HELP!!!

[rexone]

Okay, so my iMac Pro has been on Mojave for a while as there were a couple of apps I use that will die beyond it AND I have had a few bad experiences with major OS updates so, there’s that.
Anyhews… tried to update to Ventura, as was suggested by the system, and it has totally & utterly tanked my machine.

First attempt said it was going to take 10-hours & then just hung after a couple of hours before returning an error screen (see below).
Attempted a second install and it said 20-hours and then did the same.

Tried to reinstall from TimeMachine back-up and received a different error (see below)

Shut it down & restarted and got the Circle-slash screen with the support URL below…
I have followed suggestions from there and run Disc First Aid and have tried to initiate a new install but now it’s showing 2 x hard-drives AND saying there’s not enough room for the update!?!?! (see below) which is crazy as the disc should be only half-full as all my main libraries are on external drives.

How do I get my machine back to actually working?
Is there a way I can download Mojave, reboot from an install stick & do things that way?

Sort of urgent as this is my work machine.

 

[rm5]

I feel like there've been other iMac/iMac Pro users with this same issue—I remember reading (long before I joined here) about iMac Pros just randomly failing to boot after OS updates. Although since I don't own one myself, I have no first-hand experience.

When you reinstall, make sure to completely reformat your drive again as APFS to avoid any further issues. Try going into recovery > Disk Utility, clicking View > Show All Devices, and erasing the ENTIRE DISK (topmost volume). Does that fix anything?

Also, have you tried Shift + Option + Cmd + R while booting it up? That should get you to High Sierra (oldest supported OS), and you can upgrade back to Mojave.

 

[rexone]

Thanks @rm5
After much messing around I managed to get it started & reinstalled from my TimeMachine back-up but…
Now I have two hard-discs (see pic). I have no idea what has happened.
There appears to be data on both & I ***think*** I know which one is the correct one but obviously this is not a tenable set-up.
How on earth do I now get it reformatted & back to a single disc & then reinstalled from the back-up?

 

[Fishrrman]

I'm going to suggest a plan which I believe will guarantee you success.
Whether you wish to try it is up to you.
PRINT OUT this reply -- you'll need the hard copy beside you.

FIRST STEP:
BACK UP your data to an external drive (perhaps you already have a backup).
That's because we're going to COMPLETELY ERASE the internal drive and start over.
Your tm backup should be ok.

NEXT:
Boot to internet recovery.
(NOT "the recovery partition").
Command-OPTION-R
at boot.
KEEP HOLDING THIS KEY COMBO DOWN for a while.

Ethernet is best for this, but if you use wifi, you'll need your wifi password.
The utilities take a while to load, be patient as the globe spins.

WHEN THE UTILITIES GET LOADED:
Open disk utility.
VERY IMPORTANT STEP:
Go to the "view" menu and choose "show all devices".
You can't erase "the whole drive" unless you do this first !

Look at "the list on the left".
The topmost item should be the internal drive (SSD)
Click on it and click the "erase" button.
Erase to "APFS, GUID partition format".
When done, THE ENTIRE DRIVE has been wiped clean.

Now you can quit disk utility and open the OS installer.
Start clicking through.
The iMac will reboot one or more times, and the screen will go dark for up to a minute with no other indication of activity. Be patient.

When done, you should see the initial setup screen (choose your language).
CONNECT YOUR BACKUP DRIVE and start "clicking through".
At the appropriate moment, setup assistant will ask if you wish to migrate.
YES, you do, so "point the way" for setup assistant to the backup.

Give SA time to "digest" everything.
SA will present you with a list of stuff to migrate.
I recommend that you migrate it all.
So turn SA loose and let it do the migration.
When done, you should see your old login screen.

IF YOU DO THINGS MY WAY, you will end up with a PROPERLY-FORMATTED and partitioned drive (NOT like what you have now).

Good luck.

 

[CoastalOR]

OP: Don't forget that your iMac Pro has a T2 Security Chip. The default security setting does not allow booting from another drive. If you have modified the security setting to permit booting from another source other than the internal drive then that modified security setting will be lost if you format the entire drive.

Here are some references that have information about the T2 Security chip:

Mac computers with the Apple T2 Security Chip - Apple Support

Learn which Mac computers have the Apple T2 Security Chip.

support.apple.com

About Startup Security Utility on a Mac with the Apple T2 Security Chip - Apple Support

Use Startup Security Utility to make sure that your Mac always starts up from your designated startup disk, and always from a legitimate, trusted operating system.

support.apple.com

Can I back up one computer and use the clone to restore another computer? | Bombich Software

bombich.com

 

[rexone]

Hi @Fishrrman et al.
Thanks for the help & sorry for the delay.
What a totally traumatic experience...
With the steps so generously provided I was able to erase & then install. I took a gamble & did install Ventura and, apart from taking hours, it installed just fine. Then used my TimeMachine back-up to migrate everything across. That took about 5-hours...
Anyhews... now back up & running.
Thanks again for the assist.
Still have zero idea why it all went pear-shaped in the first place.