Cannot erase Macintosh internal HDD

[EdwS]

I'm currently running an iMac Retina 5k, 27 inch, late 2015 desktop with 4 external HDD's. I had previously installed OSX High Sierra BETA 10.13 ( with periodic updates ) to the Macintosh internal HDD.
When Apple announced the release of the Public version, I wanted to install that version but was put off because it was said the public version would not support the fusion drive on my system , and in effect would not allow installation on my system using the Macintosh internal HDD/Fusion Drive.

I decided to use a 3TB external drive I had installed previously, and successfully loaded the new OS and am running successfully from the external drive at present.

Wanting now to erase the old OSX Beta from the internal Macintosh drive, I have tried using Disc Utility without success. When I try, the "erase" function seems to start but nothing happens ( I've waited over an hour to see if it was just slow ) I have browsed through similar threads and tried several "Terminal" commands again seemingly without success. I have also noted that after running the Diskutil CS List command, whatever I've managed to do, has somehow erased "Core Storage". ( ? )

Any suggestions would be appreciated - as said, I simply want to erase the Macintosh internal drive so it can be used. Attached is a screenshot of the DiskUtil list commands.

 

[nambuccaheadsau]

Suggest disconnecting all externals, select one that has a bootable operating system, i.e. has a clone made bootable by SuperDuper or CarbonCopyCloner. Boot from the clone, go into Disk Utility and select the internal drive and erase and format.

High Sierra will still not run on your Fusion Drive so perhaps first download plain old Sierra and run that. The latest rumour if the next operating system, OS X.14, due around October, will function of Fusion Drives.

 

[goldbuffalo]

In Disk Utility, top left is View, drop down. Set it to Show all Devices. I was trying to reformat an external WD drive for TM back up, and that is what i needed.

 

[AZhappyjack]

Suggest disconnecting all externals, select one that has a bootable operating system, i.e. has a clone made bootable by SuperDuper or CarbonCopyCloner. Boot from the clone, go into Disk Utility and select the internal drive and erase and format.

High Sierra will still not run on your Fusion Drive so perhaps first download plain old Sierra and run that. The latest rumour if the next operating system, OS X.14, due around October, will function of Fusion Drives.

This is incorrect information. High Sierra most definitely WILL run on a Fusion drive. What will not work on a Fusion drive at the current time is Apple's new file system (APFS) that was introduced with High Sierra.
[doublepost=1527571395][/doublepost]This video is from the Carbon Copy Cloner people (Bombich Software)... it does a very good job of explaining how to partition and format a drive for macOS installation... where you seem to be getting hung up is concerning partitioning, and the concepts of volumes and containers.

 

[Fishrrman]

1. DISCONNECT ALL EXTERNAL DRIVES except for the one that's bootable.
2. Boot from the external drive that boots (probably doing that already)
3. Download CarbonCopyCloner (free to download and use for 30 days):
http://www.bombich.com/download.html
Don't run CCC yet.
4. BEFORE YOU DO WHAT FOLLOWS, YOU HAVE TO HAVE YOUR PERSONAL DATA FROM THE iMAC'S INTERNAL DRIVE BACKED UP SOMEWHERE
5. Open Disk Utility
6. Go the "view" menu on left -- make sure "Show all devices" is your choice
7. You need to click on the TOPMOST line representing your internal (fusion) drive
8. Now, ERASE IT to Mac OS extended with journaling enabled.
9. Once erased, you might run "repair disk" on it, just to be sure it's ok. If you get a good report, go on to step 10.
10. Open CarbonCopyCloner. Accept the defaults. Let it run in "trial mode" -- ALL features are functional in demo mode.
11. In CCC's window, for the source (on the left) select the external drive that you're booted from.
12. To it's right, select your INTERNAL DRIVE as the "target"
13. Let CCC do its thing and clone the external boot drive to the fusion drive.
14. When done, quit CCC and power down -- all the way off
15. DISCONNECT the external boot drive
16. Press the power on button and IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears.
17. Do you see the internal drive as a potential boot drive? If so, click on it with the pointer and hit return.
18. Do you get a good boot?

 

[nambuccaheadsau]

Should have said HS will not run in its full format using APFS and the question is why bother when this feature is not available. HS is optimised for SSDs only.

https://apple.stackexchange.com/que...-are-the-pros-cons-of-converting-hdds-to-apfs

 

[BLUEDOG314]

IMO a clean install is better here, but you need to get back to a regular core storage jhfs+ volume. I would use internet recovery, as you are past 10.12.4, internet recovery should take you to the latest compatible MacOS your machine supports. Reboot holding CMD+Option+R. Then run in terminal 'diskutil apfs list' and copy the UUID for the APFS container. Then run 'diskutil apfs deletecontainer PasteUUIDhere' which will nuke the APFS container. This should also convert the two internal drives back to jhfs+. Now do 'diskutil cs create "Fusion Drive" disk0 disk1' or you can call it something other than Fusion Drive, just use quotes if there is a space in the name. Then, 'diskutil cs list' and copy the LVG UUID, then run 'diskutil cs createvolume PasteUUIDhere jhfs+ "Macintosh HD" 100%' which will create you a blank fusion drive. Back out of terminal and reinstall the OS.

It is also worth noting that nothing says HS isn't meant to work on non SSDs. Its just the copy on write nature of APFS isn't best suited for mechanical HDDs due to the way it always writes to free space. In this respect it is similar to ZFS which is better documented at this point.