Trying to understand APFS

As Kilted Tim noted, you should not see the - Data volume mounted.

Did you install twice by any chance?

Open Disk Utility and see if there is a MacHD, MacHD - Data, and MacHD-Data.

If so, check to make sure MacHD - Data is mounted at /System/Volumes/Data and MacHD-Data is mounted at /Volumes (look at the mount point in there info pane when you select each).

You can remove the one mounted at /Volumes. You might want to make sure you didn't store anything in it before you remove it. Copy the files over to your home folder, then use Disk Utility to Remove the Volume.

Mac HD volume is reserved for the System files and Mac HD-Data is where your files and applications are stored. I'm not really sure what you are referring to on copying to Mac HD-Documents. The split volumes will appear in Disk Utility. Where are you seeing it?

You should only see Macintosh HD in the sidebar of the Finder, or on your desktop. You should have drug your documents into your user documents folder from within the finder.

Tim Murray2 wrote:

What is the point of MacHD-Data? When opened, it's (visually) empty.

Is it still empty after you put your documents into it?

Something doesn't look right here. What do you mean you "shortened the names"? You have "MacHD" and "MacHD-Data". But a normal APFS boot drive should look like "MacHD" and "MacHD - Data". Note the spaces.

Can you clarify exactly, in minute detail, what you did? All you need to have done was erase the internal drive and format it as either HFS+ or APFS (I guess). Then run the installer and target the disk. The installer would set things up the way it wants. When done, it would read "MacHD" and "MacHD - Data". There would be no "shortening".

Did you attempt to create the data volume by hand? Regardless, whatever you did, you'll need to re-do it. Wipe the drive and reinstall the OS.

You can use whatever name you want. The fact that you shortened the name has nothing to do with the problem.

You should only ever see the - Data drive in Disk Utility. The fact that you do implies you reinstalled at some point after installing Catalina or Big Sur.

And, none of this has anything to do with APFS. The split drive thing is enabled by APFS, but it is a function of Catalina and now Big Sur splitting the drive into an OS volume and a Data volume. The OS Volume is mounted Read Only to protect it from malicious modification.

You should only have one Data. Check each with Disk Utility and see how much space is on each. You can delete the empty one. Since the one under the highlighted one is the same as what you showed in the Finder, that one should be empty, except for anything you put into it.

You should be able to delete the last one.

Tim Murray2 wrote:

I simply shortened the names in Disk Utility's erase function.

names? As in plural?

I'm just going to re-do it again. In Disk Utility, I have a volume with spaces. I've been shortening the drive name for decades -- I think Catalina was "Macintosh HD" which I shortened to "MacHD".

You can name the drive anything you want. But you can't create the " - Data" portion. The OS installer does that.

Your screenshot suggests that you did try to create the data portion on your own. That won't work. Just create a drive with a single partition, named whatever you want. Install onto that drive. When you're done, you'll have a whole bunch of funky partitions.

Tim Murray2 wrote:

Should have been singular.

No. It was always plural. That was the original problem.

I am going to bet I selected MacHD > Erase, and got the option to erase just that volume or the volume group. I probably took just that volume. Then I went back, selected MacHD - Data, removed the spaces in the name, then did an Erase.

There is one way, and only one way to do this. You have to delete the entire drive - all partitions. If you are going to format your own drives, you will need to go into Disk Utility’s “View” menu and make sure “Show All Devices” is checked. Then you will be able to select the drive itself and erase it. You may need to erase any partitions and/or volumes first.

This is not something that is normally done. Apple sells new computer with the OS already installed. All you need to do is click the “upgrade” button and it handles it. But if you want to do it yourself, you will have to jump through a few more hoops and ensure you are doing it exactly as the operating system expects.

You can name the hard drive anything in the world you want. But you cannot ever change the “ - Data” portion of the name. Those spaces are significant.

You should not be seeing both of those on the desktop.

I agree with the above. You need to wipe the system and re-install from scratch. Don't try monkeying with the way macOS partitions the drives.

Did it disappear from the Finder also?