Macintosh HD volume erased and unmounted during Catalina installation on separate APFS volume

Try clicking the "Mount" button when you have the old grayed out "Macintosh HD" volume selected (aka "disk1s1"). Does it mount?

I don't know what occurred to that "Macintosh HD" system volume, but you can still access the data within the "Macintosh HD - Data" volume from your current Catalina installation. If there is only one user on each system, then the UserIDs for the accounts should be the same so you should be able to easily access that data. If the UserIDs happen to differ, then it just makes things a little more tricky is all.

You may find using Carbon Copy Cloner (CCC) very useful to making a backup of that "Data" volume and/or just your home user folder(s). CCC can access them even if the UserIDs differ because it will prompt you for admin access. Many contributors on this forum use CCC as one of their backup options. I would say get your data onto an external drive so you can be assured the data is safe. Then you can try various options to recover the system. I think CCC will make the process much simpler & quicker because you can easily grab the entire volume or home folder keeping everything intact.

You should try booting into Internet Recovery Mode (Command + Option + R) to run Disk Utility First Aid on the hidden Container as this will also run First Aid on each of the APFS volumes within that Container. Even if the First Aid summary says everything is Ok, click "Show Details" to see if any unfixed errors are listed. If so, run First Aid again until they are gone. If the errors remain after several scans, then those errors cannot be repaired and that APFS volume will need to be erased.

You do have several more options. Probably the easiest one since you have so much Free storage space is to create a new APFS volume to install Sonoma, then during initial setup migrate from the "Macintosh HD - Data" volume. If successful, then once you confirm everything is Ok, then delete the original Sonoma volumes. You can then use Disk Utility to rename the new Sonoma Volume Group as "Macintosh HD" which theoretically should rename all the other APFS volumes associated with the new Sonoma installation to they stay aligned in name.

I guess you can try to reinstall Sonoma over top of the "Macintosh HD" volume assuming it recognizes the volume as still healthy. If you install Sonoma to the "Macintosh HD - Data" volume, then it will relocate the old data to "/Users/Shared/Relocated Items" folder where you can access it, but it does leave things a bit messy including having the APFS volume names becoming "Macintosh HD - Data" for the system volume and "Macintosh HD - Data - Data" for the actual Data volume containing your home user folder (it can be renamed though, just be careful). But on initial installation & setup your home user folder will be empty.

FYI, there are many new ways to permanently lose access to the data stored on the internal SSD of the recent Macs due to all of the hardware, software, and security changes....you are lucky here since your data should still be accessible here. Once you get things straightened out, make sure to start some type of backup regimen.

dylanpryce wrote:

Just to clarify, I'm currently unable to access the Macintosh HD - Data drive from my Catalina partition (it's not listed anywhere in Finder,

Yeah, that is normal since Catalina was the first macOS to use multiple APFS volumes for the "Macintosh HD" by separating out the "Data" volume. In fact each later OS treats other boot volumes differently in their own way.

You need to look in "/Volumes" for either "Macintosh HD - Data" or perhaps just "Data". I suspect it will be the former, but it is hard to say. It is possible you may need to use Disk Utility to manually mount that volume. Plus Disk Utility will show the actual mount point for it including its name (in case it is modified due to having another volume already mounted with the same name).

You may have to use the "Choose a Folder...." option in CCC if the above does not help since technically the Sonoma Data volume may now just be considered a standard volume or even a "folder".

I tried showing every type of disk item on the desktop via the Finder preferences), likely because I had to set up my Catalina partition under a different user ID because it wouldn't allow me to get past the terms of service screen due to an unrelated bug with Catalina's setup.

You cannot be signed into the AppleID or iCloud when accepting the Terms of Service. This has been an issue for several years now. Once the Terms are accepted, then you can log into your AppleID & iCloud.

Additionally, "Update" appears to be its own volume; not linked to the "disk1s1" volume in any way.

That is because Catalina actually had a (new for Catalina) APFS volume called "Update". With later versions of macOS this has also been transformed more into an APFS snapshot than a true volume and sort of lost its actual "Update" name.

I'm not sure if there is any way to remove the icon from the Desktop without unmounting it. I've never tried to remove the "Update" volume from the Desktop.

dylanpryce wrote:

UPDATE: I actually can access the data on my Macintosh HD - Data drive straight from CCC - I just needed to right click on the drive and click "Reveal In Finder". Not really sure why I hadn't thought about doing that before haha. Thank you for the tips!

Good job! And thanks for sharing that information. I forgot about that was a possibility. Definitely makes a difference between helping someone remotely without directly working on their system & actually doing the work myself.

Please update us on how things go. It will help me for assisting others and it may help people who come across your thread.