iMac Fusion Drive or Disk Utility Issue

You didn't mention what version of Mac OS X you're trying to install, but the USB installer you created should have a copy of a CoreStorage compatible Disk Utility on it (select it from the Utilities menu displayed on the 1st screen of the installer)

A couple of things.
The PC thing is probably because there is Windows boot information on an EFI partition (which you cannot see in Disk Utility).
If you looking to completely wipe the whole machine the command line is probably the only way to be sure, you will need to do it booted from another source than the disk you wish to edit.

This is a snippet out of a reformat script I use, you should be able to work out the commands required to find and remove any existing Core Storage volumes, but let us know if you can't. If you remove everything using terminal and then bring up Disk Utility it usually prompts to repair the fusion drive, if it doesn't it actually possible to rebuild it using command line as well.

Remove_Existing_CS(){
CS_Group=$(diskutil cs list | awk '/-- Logical Volume Group / {print $NF}')
if [[ "$CS_Group" ]]; then
echo "Removing CS group detected at $CS_Group"
diskutil cs delete $CS_Group
sleep 2
else
echo "No CS group detected"
fi
CS_Group=$(diskutil cs list | awk '/-- Logical Volume Group / {print $NF}')
}

I'm using a 10.10 installer USB, but I don't see the option for CoreStorage Disk Utility? Only the one shown in the photos.

How would I run the above script using terminal on the USB? Or would I need to use Target Disk Mode and run it on my mac?

Thank you

What happens if you boot from the 10.10 USB installer, run Disk Utility from the Utilities menu, and try to re-partition the hard drive as a single partition? In theory it should re-build the fusion drive.

I think Terminal is available under Utilities as well. @sdagley is right the Disk Utility should do it automatically, fusion has been around a long time now so any recent installer would support it. What can happen however is if there is an existing Core Storage volume on the drive sometimes Disk Utility cannot remove it, hence where the command line comes in.
You should be able to do it as two parts.
Use

diskutil cs list

If CS exists this will display the details, what your after is Logical Volume Group it tends to be a great big long string of numbers and letters.
then use

diskutil cs delete THE_GROUP_FROM_ABOVE

to remove that group.

Just make sure your getting the correct one and you do really want to delete everything on the drive.

Hi All,

Thanks for the responses.

I managed to get it done by using "delete LVG" and then the group name.

Thanks all for your assistance!