My Fusion Drive Partitioning Seems Wrong/Strange/Wonky

Question

See below for diskutil list of my 3TB fusion drive (iMac 27" late 2014) running High Sierra: I assume disk0 is the HD and disk1 is the SSD, and disk2 is the virtual combo that is mounted as my "HD". But I've got a 2 questions:

1. Why does disk0 have two Apple_CoreStorage partitions? I'd like to do a clean install of Mojave for my upgrade, and if the partitioning is wrong, how do I fix? I assume erasing the disk as part of the upgrade won't change the partitioning. And partitions 2 and 4 are not adjacent, so not clear how I'd merge. And Disk Utility just shows a single 3.1TB drive. Or should I not care?

2. When I mount and look at the EFI partitions on Disk0 and Disk1, I just see an Apple sub-directory... no Windows/Microsoft anything. And the Startup Disk Utility also only shows "Macintosh HD / MacOS 10.3.6" as an option. But when I boot up holding down Option key, I see a Windows boot option. Where is this boot option hiding?

    **/dev/disk0 (internal, physical):
       #:                       TYPE NAME                    SIZE       IDENTIFIER**
       0:      GUID_partition_scheme                        *3.0 TB     disk0
       1:                        EFI EFI                     209.7 MB   disk0s1
       2:          Apple_CoreStorage Macintosh HD            2.2 TB     disk0s2
       3:                 Apple_Boot Recovery HD             650.1 MB   disk0s3
       4:          Apple_CoreStorage Macintosh HD            801.4 GB   disk0s4
       5:                 Apple_Boot Boot OS X               134.2 MB   disk0s5
    
    **/dev/disk1 (internal, physical):
       #:                       TYPE NAME                    SIZE       IDENTIFIER**
       0:      GUID_partition_scheme                        *121.3 GB   disk1
       1:                        EFI EFI                     209.7 MB   disk1s1
       2:          Apple_CoreStorage Macintosh HD            121.0 GB   disk1s2
       3:                 Apple_Boot Boot OS X               134.2 MB   disk1s3
    
    **/dev/disk2 (internal, virtual):
       #:                       TYPE NAME                    SIZE       IDENTIFIER**
       0:                  Apple_HFS Macintosh HD           +3.1 TB     disk2
            Logical Volume on disk1s2, disk0s2, ...
            F8575B66-EA8C-4260-96E8-7FEFD183EFC3
            Unencrypted Fusion Drive

CoreStorage Logical Volume Groups
|
+-- Logical Vol Group 8AAC.. 
 ===== 
 Name: Macintosh HD 
 Size: 3.1 TB 
 | 
 +- Phys Vol 1ADA.. 
 |   Index: 0 
 |   Disk: disk1s2 
 |   Size: 121.0 GB 
 +- Phys Vol 37D1.. 
 |   Index: 1 
 |   Disk: disk0s2 
 |   Size: 2.2 TB 
 +- Phys Vol CFB5... 
 |   Index: 2 
 |   Disk: disk0s4 
 |   Size: 801.4 GB 
 +- Logical Vol Family A061..
 ---
   | 
   | +- Logical Vol F857.. 
   | Disk: disk2 
   | Size (Tot): 3.1 TB 
   | Revertible: No 
   | LV Name: Macintosh HD 
   | Volume Name: Macintosh HD 
   | Content Hint: Apple_HFS LVG 
   | Type: Fusion, Sparse

Answer 1

This procedure is going to get you back to a "factory fresh" system where your 128GB SSD and 3TB (spinning) HDD are configured as a Fusion Drive. Basically, we're going to:

• Delete all partitions off all the physical drives
• Create the Fusion Drive
• Install macOS

Before you begin, ensure that you have

• all necessary backups.
• a bootable USB installer
• WiFi and a working Internet connection (if you want to use Internet Recovery)

Erase the Disks

Using a bootable USB macOS installer, boot up then go into Terminal and issue the following commands:

$ diskutil partitionDisk disk0 1 GPT HFS+ MacSSD 100%

$ diskutil partitionDisk disk1 1 GPT HFS+ MacHDD 100%

Verify that the disks are wiped clean by issuing the command

$ diskutil list

You should see two disks with a single partition each.

Create the Fusion Drive

Create the Logical Volume Group

Next create a logical volume group by "combining" the two drives with the following command

$ diskutil coreStorage create <LogicalVolumeGroupName> /dev/disk1 /dev/disk2

You can use any descriptive name for the group. Ex: iMacLVG

When the process has finished, it will output some status information including the logical volume group UUID like below:

Core Storage LVG UUID: XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXXX

Copy that number to your clipboard. You will need it for the next command.

Create the Logical Volume

Issue the command

$ diskutil coreStorage createVolume lvgUUID type name size

Where...

• lvgUUID = the UUID you copied from the last step
• type = the format you want to you.
• name = the name of the Volume.
• size = how big you want to make the volume. Valid entries are in bytes (Kilo, Mega, Giga, Tera, Peta, etc.) and percentage (%)

So, to make a volume that spans the whole logical volume in HFS+ use the following example:

 $ diskutil coreStorage createVolume  XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXXX HFS+ "iMac HD" 100%

Once it's done, you've got a clean volume on which to install macOS. Issue the command to verify that you've got 3 disks: 2 physical and 1 logical.

Install macOS

Exit Terminal and complete the macOS installation. Restore your data by using the Data Migration Tool if necessary.