Fusion drive after formatting - becomes three times less

Hi Light_Knight,

Thanks for pausing here. Although your smaller SSD (disk1) seems to be ok, your larger HDD (disk0) is only reporting 1TB used by partitions even though it claims it is 3 TB large:

Normally, all 3 TB should be used up in partitions. You should first erase the HDD in Disk Utility before proceeding:

While booted into Internet Recovery Mode:

1. Quit Terminal and open Disk Utility.
2. Click on the View button, then select Show All Devices.
3. Eject and disconnect all external drives to avoid erasing them (such as your Time Machine backup disk).
4. Look for a device named "APPLE HDD". Select this device and click Erase. Format as follows:
   1. Name: Whatever you want
   2. Format: Mac OS Extended (Journaled)
   3. Scheme: GUID Partition Table or GUID Partition Map
5. Click Erase. When the erasure is complete, check the amount of drive space reported by Disk Utility. If it's 3 TB, you're good to go and can safely continue.
6. Quit Disk Utility and open Terminal again.
7. Execute diskutil list to display a list of all drives (disks) connected to your Mac. The disks you're looking for should have a capacity of (close to) 3 TB and 128 GB, respectively.
   1. In the sample output you've posted, this would be disk0 and disk1.
   2. Verify that the 3 TB disk is showing all 3 TB used up in partitions. If it seems like only 1 TB is being used up in partitions, stop here and try erasing the HDD again.
8. Assuming that your internal drives are disk0 and disk1 (as shown by your sample output), execute the command below:

   diskutil cs create Macintosh\ HD disk0 disk1

9. Once the operation is complete, execute diskutil cs list. Copy the string of characters that appears next to "Logical Volume Group".

   To copy, press Command-C.

10. Execute the below command. Replace lvgUUID with the string of characters you copied in the previous step:
    1. diskutil cs createVolume lvgUUID jhfs+ Macintosh\ HD 100%
    2. To paste, press Command-V.
11. Your Fusion Drive is now rebuilt. Quit Terminal and select Install macOS to reinstall the operating system onto your Mac.
12. When the installation is complete, Setup Assistant should appear. Progress through the Assistant until you're prompted to transfer data. If you have a Time Machine backup, connect your backup drive and select to transfer "From a Mac, Time Machine backup or startup disk". You'll be guided through the data restoration process.

Hi Light_Knight,

The reasons why you cannot erase the top level of your internal drive are listed below:

• Erasing the top level of a Fusion Drive isn't supported, but the equivalent of this can be done by dismantling the Fusion Drive in Terminal.
• You can't erase the top level of an internal drive if you used Command-R to boot local Recovery Mode. Otherwise, the currently booted Recovery system would be wiped out in the process.

To reclaim the 3 TB storage, you'll need to dismantle your Fusion Drive and then rebuild it. This process will erase all partitions and data on your Mac. Ensure that you have a Time Machine backup before proceeding.

1. Boot Recovery Mode over the Internet by holding down Option-Command-R as you turn on or restart your Mac. Release the keys when a spinning globe appears, or when you're prompted to connect to a Wi-Fi network.

   If you don't see a spinning globe but instead see the Apple logo immediately, you haven't booted Recovery Mode over the Internet.

2. When the main Utilities window appears, don't select any utilities presented. Instead, go to the top menu bar (where the Apple logo resides) and click on Utilities. Then, select Terminal.
3. Execute diskutil cs list. Select the string of characters that appears next to Logical Volume Group, and press Command-C to copy.
4. Execute diskutil cs delete lvgUUID (replace lvgUUID with the string of characters you copied in the previous step) to dismantle your Fusion Drive and erase all data stored on it.

   To paste, press Command-V.

5. Follow the steps in this article to rebuild your Fusion Drive and install macOS: How to fix a split Fusion Drive - Apple Support.

Ok

Before you start be sure you have a good up-to-date backup (preferably two different backups on two different drives)

Disconnect any external storage devices that are connected to your Mac.

Boot into Recovery mode. From the  menu, choose Restart. As the Mac starts up again, hold down Command + R keys.

Release the keys when you see the Apple logo or a spinning globe.

After a minute or two, you should wind up at the language Select menu, select the one you want and click continue. You will now see the Mac OS Utilities Window.

Select Disk Utility from the utilities window of macOS Recovery, then click Continue.

under view select show all devices and select the top one

Click the Erase button or tab

Complete these fields:

Name: Enter a name for your disk, such as "Macintosh HD".

Format: Choose Mac OS Extended (Journaled) or APFS.

Scheme (if available): Choose GUID Partition Map.

Then click Erase wait for it to finish

When it is done quit disk utility by pressing the Command + Q keys or by choosing Disk Utility > Quit Disk Utility

Choose Reinstall macOS (or Reinstall OS X) from the utilities window.

Click Continue, then follow the onscreen instructions. You'll be asked to select your disk. If you don't see it, click Show All Disks.

After macOS installation is complete

you can use migration assistant to transfer the files from your backup

Hope that helps, have a nice day

Hi,

This article might help.

How to fix a split Fusion Drive - Apple Support

migration assistant