Macintosh HD or Macintosh HD - Data?

Usually one can't see the System in Finder but sometimes after a failed install there can be a duplicate after a subsequent install.

Please post a screenshot of what disk utility shows. Make sure that View All Devices are selected in the View menu and that all drives are fully expanded, that is all carets ⌵ point down.

To take screenshot

Take a screenshot on your Mac - Apple Support

To add it to a reply

Click on the add image icon right below the test entry box

fter clicking on the icon you are present with a Finder like windows. maneuver to find the image file you want to post and click on it and select Open in bottom right or double click on the image file to add the image in the text reply box.

As mentioned by previous poster regarding Macintosh HD and Macintosh HD - Data. The Macintosh HD is not only read Only but it is a Sealed and Read Only Volume.

The Macintosh HD Volume can not be altered or modified by Third Party Software or the User.

This was by Apple Design to further insure against security breaches or accidentally/ intential attempts to modify the OS.

You don't. And actually, they already share the same space on the drive.

What Illaass is getting at is something we've seen here multiple times. A drive was improperly erased (or whatever) and instead of just one Data drive, there are two. One hooked to the current System volume, and the other a dead leftover. That may be what you're seeing, and a screen shot of Disk Utility would show.

Time Machine does not know which one to back up. Can i just rename one to make it work?

I'm not certain, but that sounds like you have an orphan Data drive from a previous installation.

It is easy to tell if there is an orphan if you use Finder Go > Computer and you see a volume named, "Macintosh HD - Data."

Open Disk Utility and look for two Volumes named Macintosh HD - Data. One will be inside the "Macintosh HD volumes" group. The other will be separate from the startup volume and current data volume.

Select each and look at the info pane. The current Data volume will show a House badge on the icon. The mount point will be /System/Volumes/Data.

The orphan will not have a house badge and the mount point will be /Volumes.

If that is the case, select the orphan and click the Remove Volume button in the Toolbar (–).

It is ok to rename your startup drive to whatever you want, but that won't solve your problem. The data volume will rename to match (but you will never see it).

Yeah, I figured it out. Somehow the upgrade installation borked the volume configuration. I renamed the old volume in Disk Utility so I could tell them apart, copied what I wanted to keep from it to the current Data drive, and then deleted the old one. Seemingly no harm done otherwise.

Do not touch the names. The main HD volume is read only, and where the OS is located. Data is where your user account is. You also shouldn't even be seeing the Data volume on the desktop.

Any suggestions on how to merge the two drives? I just installed Monterey onto a machine that had Catalina, and I have the same problem: the System and Data drives show up separately in the Finder…

Yup, sounds like you got it. Pulled necessary files out of the orphaned volume, and then canned it.