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:
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