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Long story short. On my iMac I had a Fusion Drive: 250GB SSD + 750GB HDD.

The SSD failed totally, is completely dead. HDD seems to be OK. I replaced the SSD and erased the HDD.

The HDD seems to be OK. I can use it normally and Disk Utility doesn't show any problems. Only it shows the wrong (old) size (of the Fusion Drive) - still 1TB:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Data                    999.9 GB   disk0s2

I am afraid to use this drive as it shows more space then there is physically.

Is there a way somehow to reset this drive totally so I can use it again? Standard utilities like diskutil and fsck_hfs exit without any errors.

EDIT: Answering the comment:It is definitely a 750 GB drive. iMac didn't want to boot from the drive and from a bootable USB either. I had to disconnect both drives - then I could boot form the USB. I replaced the SSD and installed new system on it. Disk Utility was showing the HDD as an incomplete Fusion volume:

Checking volume

disk0s2: Scan for Volume Headers

disk0s2: Scan for Disk Labels

Logical Volume Group 56E4E02A-D9B3-462B-BA0A-6E9A9D4D9646 spans 2 devices

Incomplete or inconsistent CoreStorage Physical Volume set

Storage system check exit code is 1.

Problems were found with the partition map which might prevent booting

Operation successful.
  • Are you certain it's not a 1TB drive? A Core Storage drive would say /dev/disk2 (internal, virtual) Can you post the output of the command system_profiler SPStorageDataType just limit it to the "physical volumes" – Allan Sep 8 '17 at 11:48
  • Yes. It is definitely a 750 GB drive. iMac didn't want to boot from the drive and from a bootable USB either. I had to disconnect both drives - then I could boot form the USB. I replaced the SSD and installed new system on it. Disk Utility was showing the HDD as an incomplete Fusion disk0s2: Scan for Volume Headers disk0s2: Scan for Disk Labels Logical Volume Group 56E4E02A-D9B3-462B-BA0A-6E9A9D4D9646 spans 2 devices Incomplete or inconsistent CoreStorage Physical Volume set Storage system check exit code is 1. Problems were found with the partition map which might prevent booting – Pawel Sep 8 '17 at 11:56
  • Then I Erased this disk and the result you see in my post – Pawel Sep 8 '17 at 11:57
  • I will post the output as soon as I am at the computer. – Pawel Sep 8 '17 at 12:04
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The easiest way to fix this is to boot using your bootable USB, go into terminal and enter the following command to (re)partition your drive

diskutil partitionDisk /dev/diskX 1 GPT HFS+ MacintoshHD 100%

Where X is the number of the drive that corresponds to your HDD. Issue the command diskutil list to get a listing of the disk identifiers.

Verify that your drive is showing up correctly by issuing the command

diskutil info diskX

Then proceed with your installation of macOS

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