1

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
  • There's a lot to unpack here.... Starting with the partitioning of a drive for a clean install..... If you're doing a complete wipe of the system and want to destroy all the partions, you can use the command found in this answer. Just do that to all your disks to ensure they're blank. (Make sure you have a working method to reinstall macOS – Allan Mar 4 at 15:23
  • Next, it seems you have some (possibly) previously created core storage volumes. Issue the command diskutil coreStorage list and post that output as well. Finally, when you boot into Windows, do you get a full Windows environment? – Allan Mar 4 at 15:24
  • 1
    Please add that to the the original post by editing the question. It’s next to impossible to read that especially on a mobile device – Allan Mar 4 at 16:13
  • So if I boot from USB, make sure disk0 has no mounted partitions, and use the command 'diskutil partitionDisk disk0 1 GPT HFS+ MacHD 100%' it seems I'd get a "clean disk". But that spawns two new questions: 1) Seems like I'd lose my "Apple_Boot Recovery HD" and "Apple_Boot OS X" partitions (not sure why I have two Apple_Boot partitions), and 2) Will the virtual Fusion Drive (disk2) which is a logical addition of coreStorage partitions on disk0 and disk1 "figure out" what I'm doing, reconfigure itself appropriately, and continue to function properly? – P Dolch Mar 4 at 17:53
  • As to your questions: 1) Your recovery partition will be recreated with the fresh install of macOS 2) No. It won't "figure out" anything and it won't function. If you're doing a complete wipe as you alluded to in the original question, then you want to wipe all of your disks clean and start with a clean slate. – Allan Mar 4 at 20:38
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.

  • Thanks for laying this all out so clearly. It appears that the Mojave version of 'diskutil' now has a 'resetFusion' option, which claims to do all of the steps for you (repartition with GPT maps, create Core Storage Fusion Drive volume, etc.). I think the above steps are for pre-Mojave 'diskutil'. Thoughts on using the resetFusion option instead? – P Dolch Mar 5 at 18:07
  • I haven't gone to Mojave yet because I'm always leery of new OS releases (all vendors). I have it on a machine I "play" with, but nothing I do actual work on. I haven't tried out the "resetFusion" function, but since you're willing to wipe the drives anyway - I say give it a go. If it doesn't work, then you have these instructions to fall back on. To put it another way...you won't break anything! – Allan Mar 5 at 20:21
  • A final thanks to @Allen here since, as I'm new to the platform, I'm not yet allowed to upvote his answer. After an extra set of backups to Dropbox and to a spare external drive, I'll give it a try. If the upgrade process is successful, I'll add a final comment so that folks know the "resetFusion" option works. – P Dolch Mar 5 at 21:11

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