After failed Big Sur upgrade and manual High Sierra installation, I have two disks : "Update" and "iMac_SSD - Data". How do I fix this?

[jaxx67367346]

Hi guys,

I am coming to you to ask for your help.

I own an iMac (Retina 4k, 21.5-inch, late 2015
Processor 3.1 GHz Intel Core i5
Memory 8 GB 1867 MHz DDR3
Graphics Intel Iris Pro Graphics 6200 1536 MB)

It must be said that 4 years ago, I brought my iMac in a specialised store, where they changed the main drive (which was slow and sometimes unresponsive), to an SSD drive.

Because I am stupid, I’m unable to remember exactly which OS I was running before the problem happened.

Anyhow, basically, at some point, in a rush leaving for work and trying to turn off my computer, I accepted some update: upgrade to Big Sur.

After that, I was unable to restart my computer: it gets stuck at the loading screen (I tried leaving my iMac on for 48 hours, just to make sure it wasn’t the installation that was taking some time). I tried recovery mode (first aid on both the drive, which I think were iMac_SSD and iMac_SSD - Data ; reinstalling big sur) but to no avail.

I created a bootable installer with macOS Catalina. Unfortunately, I was unable to install Catalina (the iMac_SSD wouldn’t appear in the disk I could install Catalina on). I read somewhere that it was because I needed an APFS-compatible OS.

I created a new bootable installer with macOS High Sierra. Which - sort of - worked. I got asked if I wanted to install High Sierra on the disk “Update” or the disk “iMac_SSD - data”.

I chose the "Update"-disk and it installed High Sierra. Now, I see in the finder “iMac - SSD - Data”. I managed to save most of my data from “the iMac_SSD - Data” drive, on an external drive.

Now I am wondering what to do. How do I reset everything and do a fresh install? I feel like the Big Sur upgrade split my disk in two, with this weird “Update” disk, and the other “iMac_SSD - Data”.

If I go in the Disk Utiliy, I see two internal disks:

"Update" (1.05 TB ; "shared by 5 volumes"), which has 11.85 GB used, 703.36 GB other volumes, 337.45 GB Free. Screenshot 1 ; Screenshot 2

"iMac_SSD - Data" (1.05 TB ; "shared by 5 volumes"), which has 681.57 GB used, 33.63 GB other volumes, 337.45 GB Free. Screenshot 1 ; Screenshot 2

Is there anyway to fix everything while keeping my data? If yes, how?

If not, how do I reset everything ?

Many thanks if you can help me, this would be greatly appreciated!

 

[Fishrrman]

You have High Sierra installed now?
If so, you didn't install it correctly.
The disk structure is still at least partially "stuck back" with the Monterey install.

WHAT YOU NEED TO DO:
You're going to have to:
1. ERASE the drive first
and then
2. Re-install the OS.

I would recommend Mojave instead of High Sierra, but in any case:
a. You'll need a bootable USB flashdrive with a copy of the OS installer on it. It should be 16gb or larger in size (not sure if an 8gb flashdrive will work). You still have the one you made previously, right?
b. Boot from the flashdrive.
c. Once the flashdrive is booted, QUIT the OS installer and open disk utility
d. IMPORTANT STEP: go to disk utility's "view" menu and choose "show all devices"
e. Now, look at "the list on the left". The TOPMOST item should represent the physical SSD inside.
f. ERASE IT to APFS, GUID partition format.
g. When the erase is done, quit disk utility and re-open the OS installer.
h. Begin clicking through. The Mac will restart one or more times, and the screen may go dark for a minute or longer with no other indication of activity. BE PATIENT.
i. When done, you should see the initial screen "choose your language". Begin "clicking through" the install.
j. At this point, you have a choice to make. You can either start completely fresh (new account), OR, you can wait until setup assistant asks if you wish to migrate from another drive (such as a backup).
k. If you wish to migrate, connect your backup and give setup assistant time to "digest everything". Then, make the choices from "the checklist" and let setup assistant do its thing.
l. When done, you should see your old login screen again.

I suggest you PRINT OUT this checklist and follow along.
Good luck.