Two Macintosh HD Data disks, one is taking up so much storage. Can I delete it?

You will need a Time Machine backup to do this, so if you don't, do something else. If you want Mojave, you need a backup you made while running Mojave. If you have made any files since you installed Catalina, copy them to an external drive because the backup won't have them. If you added photos to your Photos Library, export them to the external drive.

Oh ok by "erase the disk", which disk do I erase: the data or the data2, OR the Macintosh HD one?

By erase the disk, I mean erase the disk, not a volume, not some volumes. Erase the disk.

Boot into Internet Recovery (cmd-opt-R) and Erase the entire disk, then reinstall macOS. If you want Mojave, use cmd-opt-shift-R for getting into Internet Recovery.

How to reinstall macOS from macOS Recovery - Apple Support

And what do you mean by migrate from my backup during startup assistant?

After installing the OS, when it reboots, the Startup Assistant runs. It will ask if you want to copy your data from another Mac or from a backup. Use your backup. You do have a Time Machine Backup, correct?

How to move your content to a new Mac - Apple Support

If you have a backup of when you used Mojave, you can just restore from that after erasing the disk. Use the Restore from Time Machine backup after quitting Disk Utility. You won't need to use the Migration Assistant.

Hey again! Yes, you can create a bootable installer after downloading Mojave:

create a Bootable installer:

https://support.apple.com/en-us/HT201372

Download Mojave (Under section 4):

https://support.apple.com/en-us/HT210190

Or like said above, you can boot to internet recovery, erase/reinstall the original OS, then download Mojave again:

(Boot up using Shift Command Option R):

https://support.apple.com/en-us/HT204904

A 16gb thumb drive is usually enough to create the installer, even 8 might work but preferably 16-32, otherwise using internet recovery you shouldn’t even need to create one, but they really do come in handy. Hope it works out!

Sounds like you reinstalled on the wrong volume so it created a new - Data volume.

You should just erase the disk, reinstall macOS, then migrate from your backup during the Startup Assistant.

Macintosh HD holds the OS.

Macintosh HD - Data holds your data.

If you installed on Macintosh HD - Data, it would create another data drive.

Macintosh HD contains the new copy of the OS

The first Data disk holds the old copy of the OS

The second Data disk has all your files.

Delete the first Data disk - Macintosh HD - Data

Rename Macintosh HD - Data2 to Macintosh HD - Data

Keep your fingers crossed and reboot. If this doesn't work, then you will have to start from scratch. Try to backup your data, if you can.

Hopefully, Mojave is on Macintosh HD but there is no way to know from what's available. The - Data2 entry has nearly 67GBs of data on it which, I assumed, are your files. That left the - Data entry which looks to be the old OS. When it comes to this type of problem, it's hard to be sure you are right. I think I'm right but even if I'm wrong, you won't be deleting your files.

If you ever do this again using Mojave or Catalina, then be sure to backup your files first. Then use Disk Utility to remove all the APFS volumes before erasing the drive and installing macOS. That will avoid the problem you had. You will have to follow this once you upgrade to Catalina and beyond where it is a bit more difficult and confusing.

Hey there!

Thanks for the screen shots! As in the picture you posted:

If you are logged in to your normal user, and can see all of your data as normal, (If any), you should see “Volumes” above the disks with both a “+” and a “-“ sign.

If one of the Macintosh HD Data volumes allows you to click the “-“ minus sign, that means you aren’t booted up from that volume, and if you currently have all your data while logged in, or even on an external drive, you can safely select the minus sign to remove that volume.

This would restore your storage to what it should be.

Otherwise, if you are NOT showing on Catalina, as Kappy said erasing the disk from recovery mode, (Make sure to select View > Show All Devices and select the main drive above “Container Disk” and to use APFS format etc.

But if you are able to remove then volume via Disk Utility while logged in, that should be the end of it, hopefully a quick fix, otherwise you can just do the factory reset again as said cheers!

Okay just saw it was on Mojave so, if still showing Mojave OR Catalina, like I said, I’d still just remove the Mac Data volume that allows you to press “-“, and that’s that, after you can upgrade to Catalina on the one remaining volume that your booted from now. Otherwise if issues, as said I’d just reformat it from recovery mode from the main disk (Typically AppleSSD/HDD/Fusion drive), good luck.

If the drive is erased, then the OP will lose the data or end up with yet another - Data volume.

Don't confuse the process, please. I'm fairly certain that the OP should follow what I suggested and which you concurred.

Hey again! So both the “Data” volumes are mounted? And this is while logged in to your admin account, and not in recovery mode?

Okay then, if booted to one of the volumes that would be strange.

Okay, then as Kappy said I’d reformat the disk from recovery mode and reinstall the OS, not sure what may have went wrong exactly, but you can’t go wrong doing that if you have all backed up, good luck!

To erase and install Catalina from the get go, (As you initially did), you can boot up your Mac to internet recovery with Command Option R. To be sure to remove all un needed volumes, in Disk Utility, click one of the Mac Volumes and on the top menu be sure to select View > Show All Devices and erase the main HDD/SSD/Fusion Drive, (Whichever is above the Container Disk, which should be there if in a newer recovery system), and choose APFS as the format before erasing. I find that more efficient than just removing a volume in case there may have been any formatting issues to begin with. Hope all goes good, cheers!

That is correct. Try that before your try anything else. If this works, it will save you a lot of trouble and time. In case I did not mention this prior, just select the - Data entry, CTRL- or RIGHT-click on the selection and select Delete APFS volume from the context menu. Quit Disk Utility and reboot.

Remember, do not remove Macintosh HD or the - Data2 entry which is the third entry in your earlier pics.

If you don't need any documents or other files, you don't need to backup anything. Why do you think you need to back up Mojave?

Just erase the disk completely and install the OS you want.

You can create a bootable USB from a Mojave installer, or use Internet Recovery to install the OS that shipped on your Mac, then upgrade it to Mojave.