How to fix iMac hard drive issues caused by a child's actions?

In the process of the erasing and reinstalling the macOS, you split the iMac's Fusion Drive.

What you need to do now is start over.

First fix the split Fusion drive, as per > How to fix a split Fusion Drive - Apple Support

Then Reinstall the macOS, as per > How to reinstall macOS - Apple Support

Jimg81 wrote:

Hi

My Beautiful 5 year old Niece found my unattended iMac and decided press every button, open most applications and files.

What ever she did my hard drive is now a mess,

I highly recommend you lock the computer when you walk away from it especially when your niece is around. You can enable a Hot Corner action to do this or you can use the key combination Command + Control + Q, or select the "Lock Screen" option from the Apple menu.

And if you want her to be able to use the computer, then create a Standard user account which has no ability to modify anything system wide. If she destroys that standard user account, then you delete it & create a new one.

It appears that your iMac shipped with a Fusion drive for startup and storage.

A Fusion drive is two physical drives that the OS sets up to appear as a single drive. One is a small capacity SSD and the other is a larger capacity HDD - 28 GB & 1000 GB in your case. When this fusion becomes "split" the drives appear as separate drives and the performance of the Mac suffers.

A split Fusion drive can be repaired at home using commands in the Terminal application. The process fuses the two drives again and then finally you'll reinstall macOS and your data.

This is Apple's guidance: How to fix a split Fusion Drive - Apple Support

You're welcome.