This question comes from this question: Cannot install bootcamp because of disk error, but disk utility says it's fine?

I was advised to ask about the 296.6 GB of free space, but tbh I don't understand what that means, it doesn't seem to appear in disk utility?

Is this normal and can I recover this space? Thanks.

iMac:~ aaa$ diskutil cs list
CoreStorage logical volume groups (1 found)
+-- Logical Volume Group AB41694E-348C-45DE-95E8-E64914D40975
    Name:         Macintosh HD
    Status:       Online
    Size:         2120423165952 B (2.1 TB)
    Free Space:   296568467456 B (296.6 GB)
    +-< Physical Volume 9066D3CD-9CBE-456C-B34E-D70E61FC73E0
    |   ----------------------------------------------------
    |   Index:    0
    |   Disk:     disk0s2
    |   Status:   Online
    |   Size:     120883990528 B (120.9 GB)
    +-< Physical Volume F9A29CF9-E84F-4FBA-93DF-83878D387D95
    |   ----------------------------------------------------
    |   Index:    1
    |   Disk:     disk1s2
    |   Status:   Online
    |   Size:     1999539175424 B (2.0 TB)
    +-> Logical Volume Family F9AB59F1-0C5B-4FE4-837F-96D40DCB94A6
        Encryption Type:         None
        +-> Logical Volume C4D9F5AA-BA5F-4286-A630-FE8CAF4C913F
            Disk:                  disk2
            Status:                Online
            Size (Total):          1817999966208 B (1.8 TB)
            Revertible:            No
            LV Name:               Macintosh HD
            Volume Name:           Macintosh HD
            Content Hint:          Apple_HFS
            LVG Type:              Fusion, Sparse

CoreStorage is a Logical Volume Manager. It's based on Physical Volume(s) on the physical side and assigns Logical Volume(s) on the "logical side", which can be formatted with a file system.

Your Physical Volumes are disk0s2 and disk1s2 (which itself are slices of the physical disks disk0 and disk1).

The total size of your Logical Volume Group is 2120 GB. Only 1818 GB are assigned to a Logical Volume (disk2/Macintosh HD/JHFS+). So 302 GB of the Logical Volume Group are "free space". Some of this free space is reserved for management purposes. The net free space in the LVG is 296.6 GB or less.

You now can either expand the existing Logical Volume disk2 to fill the LVG or add a 2nd Logical Volume.

Check the disks first:

diskutil verifyVolume disk0s2

To resize a Logical Volume you have to execute the following command in Terminal.app:

diskutil cs resizeVolume lvUUID size
  • with lvUUID: the UUID of the Logical Volume (in your case C4D9F5AA-BA5F-4286-A630-FE8CAF4C913F)
  • and size: a size in (t|g|m|k|b) e.g. 2114g or magic sizes like R or 0 (which should resize the logical volume to the possible max).

In my opinion you already tried to resize the LV or LVG in the past and Boot Camp Assistant cannot cope with it or some error occurred while the BC Assistant was trying to shrink the LVG by 300 GB to free up space for the Boot Camp partition.

Usually this can't be fixed and you have to recreate the Fusion Drive from scratch and restore your whole system from a backup.

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