Mac OS Mojave Macintosh HD - Data

Your mac must have at some time been running either macOS Catalina or macOS Big Sur

and you have erased it and reinstalled macOS Mojave.

When Apple introduced macOS Catalina they introduced the split Disk with Macintosh HD and Macintosh HD - Data.

macOS Mojave does not use this configuration.

When you reinstalled macOS Mojave you did not erase and reformat the disk properly.

I take it you have a bootable USB installer with macOS Mojave on it.

What you should have done to prepare your mac correctly for downgrading to macOS Mojave

is boot to the bootable USB installer.

Open Disk Utility, and this is the bit you missed, click on View in the

menubar and select Show All Devices. This will now show all the Disk and its indented Volumes.

From there you highlight the Disk (probably Apple SSD…) not any of the indented Volumes

(Container Disk, Macintosh HD and MacintoshHD - Data) then click Erase.

Give the Disk a name.

Format: APFS

Scheme: GUID Partition Map

Click Erase.

When Done quit Disk Utility.

Your mac is now correctly formatted for installing macOS Mojave.

Disk Utility will show the Disk and one indented Volume (the name you give it earlier)

which is correct for macOS Mojave and earlier OS's.

Click on Install OS and press Continue.

the installation process should start, follow the prompts.

Yes you could boot to the Recovery HD, Restart the mac while pressing and holding theCommand and R keys.

From the Utilities window select Disk Utility, click on View in the menubar and select Show All Devices.

Highlight the Disk and click Partition see if Macintosh HD and Macintosh HD - Data are seen as separate partitions.

If they are then click on Macintosh HD - Data and press the minus button and see if it releases that partition

to the Macintosh HD partition. My thoughts are that you cannot do this as the Partition feature will see the

two Volumes as one partition.

Similarly I think if you were to highlight the Macintosh HD - Data and click Erase it will erase whatever is on that Volume

but not sure if it will merge the Volume into a single Macintosh HD, or leave them as two separate Volumes.

The safest bet is to do a full Erase and reinstall Mojave and then move your data back on to the newly formatted Disk.