iMac Fusion Drive or Disk Utility Issue

New Contributor II

Hi Guys,

I have a 27" iMac here that I am trying to wipe and re setup from scratch.

I've created a USB via DiskMakerX and booted up, when I go into Disk Utility, I see the "main" hard drive partition and then 2 more hard drives underneath it. One is at 3TB and one is 17GB. (This is a fusion drive if that helps).

I cannot repartition this into one drive as it throws up an error message.

When you install an OS on the 3TB hard drive, I can browse the 17GB hard drive as if its just another internal disk. There is nothing on it.

I erased the two drives back to Mac OS Journaled, and then turned the iMac off and turned back on. I then randomly get a windows screen saying "Your PC needs to be recovered". I can only assume somebody has installed Windows on this before and now Disk Utility cannot find it those files.

My last attempt was to put the Mac into Target Disk Mode and format via my Mac, I have done this but I get the following error on my Disk Utility -

'A CoreStorage logical volume group with more than one logical volume has been detected. The Disk Utility GUI does not support full CoreStorage editing. To use Disk Utility use diskutil to revert to single logical volume per logical volume group."

I have googled this to not much avail.

Please can anybody offer some advice on what to do next!


PS - Excuse the dirty screen, its been moved around a lot to get to the ports at the back!


Esteemed Contributor II

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)

Valued Contributor III

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.

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 $CS_Group
sleep 2
echo "No CS group detected"
CS_Group=$(diskutil cs list | awk '/-- Logical Volume Group / {print $NF}')

New Contributor II

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

Esteemed Contributor II

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.

Valued Contributor III

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.

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.

New Contributor II

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!