To add to the above. hdd is 1tb fusion drive. disk0 is the SSD -128GB disk1 is the  HDD - TB and disk2 is the synthesised disk. when you run diskutil  verifyVolume or repairVolume. at each volume level say disk1s1 or disk2s1, disk2s2 etc No errors are reported for "Verifying allocated space" but when you run for the container disk like. disk0s2 or disk1s2. only then the "Verifying allocated space" it gives all those errors  warning: MT mapping (0x40000007faf09 -> 0x28aaa3, 3, C) is not completely referenced warning: MT mapping (0x40000007fd997 -> 0xfd059b, 2, C) is not completely referenced warning: MT mapping (0x4000000813aa7 -> 0x2d9f5a, 3, C) is not completely referenced warning: MT mapping (0x4000000813bb2 -> 0x2da2c6, 3, C) is not completely referenced so diskutil list /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_APFS Container disk2         1000.0 GB  disk0s2 /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_APFS Container disk2         121.1 GB   disk1s2 /dev/disk2 (synthesized):    #:                       TYPE NAME                    SIZE       IDENTIFIER    0:      APFS Container Scheme -                      +1.1 TB     disk2                                  Physical Stores disk0s2, disk1s2    1:                APFS Volume imacOSCatalinaX15 - ... 37.2 GB    disk2s1    2:                APFS Volume Preboot                 87.5 MB    disk2s2    3:                APFS Volume Recovery                529.0 MB   disk2s3    4:                APFS Volume VM                      12.9 GB    disk2s4    5:                APFS Volume imacOSCatalinaX15       11.2 GB    disk2s5 Any one has any idea... it is driving me crazy, which at the end is going to make me go and buy the new M1 Mac-mini or the M1-imac. this should just be software issue, rather than hardware issue, as apple diagnostics checked and said no errors. Thanks.`