Upgrade early 2012 iMac to Mojave?

Mojave requires iMac (Late 2012 or newer).

I’m not aware of an early 2012 iMac model. Early 2013, maybe?

Can you confirm the model year?

If you have tried the upgrade, were there any errors or diagnostics?

The 8 GB hard disk configuration is slow: Why is my hard disk drive iMac so slow? - Apple Community

Widefella wrote:

Hi Mr Hoffman I have tried too upload Mojave but it wont instal because the disk is being used by time machine . I do have time machine running independently , any ideas ?

Are you booted into Safe Mode, as described in that article? Your boot disk shouldn’t be in use by Time Machine (snapshots aside), and most of what is installed shouldn’t be active when the Mac is booted into Safe Mode, so I’m not quit sure where you’re trying to install Mojave, or what is happening with this case, uf uou are booted into Safe Mode.

Safe Mode boot aside, did you set up Time Machine on your boot disk with disk partitioning? (That’s not generally recommended.)

That’ll run Mojave.

Back up, and try the upgrade.

See the Related Article linked above.

If there are errors or diagnostics shown by the upgrade (if it fails), post those.

Image reposted sans serial:

Next steps to try: If an error occurred while updating or installing macOS - Apple Support

FWIW: When you have a screenshot in progress, place the cursor over the window you want to capture, tap the spacebar, the window to be captured will highlight, and click to capture. This’ll select and capture just the window under the cursor.

You have an iMac late 2012. You are attempting an upgrade. The upgrade is reporting issues with the volume. You have never previously had issues. You are now working through the troubleshooting steps. You will now attempt a Safe Mode boot, and an upgrade while booted in Safe Mode. If the upgrade while booted in Safe Mode should fail, you will ensure at least one and preferably two backups, preferably one of those two backups then completely disconnected, create bootable media from the downloaded Mojave installer, boot that media, followed by an erase and re-install of Mojave, and using Setup Assistant to migrate in your contents (again, not restoring your backup here, but migrating your contents) during the Mojave initial setup.

De nada.