Fusion drive only shows the SSD partition Ok, I am at my wit's end.  Last week after 10.13.3 failed to install twice, my late 2014 5K Retina iMac became painfully slow.  Eventually it just bricked.  I tried to restore from Time Capsule over ethernet but it would get to around 28% and slow to a crawl.  Like 200 hours crawl.

After numerous attempts at this I bailed and tried reinstalling the OS.  No dice there, it would just stall at the startup screen.

Next I installed macOS to an external drive and then using SuperDuper tried cloning it back to the iMac.  This worked and I ran Migration Assistant which looked promising but again, stalled around 1/3 through.

Canceled that and now when I go into Disk Utility I see this for the internal drive:


APPLE SSD SD0128F Media (Solid State) 121GB
Macintosh HD (CoreStorage Logical Volume) 120GB


The Fusion drive is supposed to be 3.11TB, I seem to have lost almost all my storage space.

I suspect this is why all my efforts to restore have failed, it gets to the new available capacity of 120GB or 240GB and doesn't know what to do.

I have not been able to find a solution to reclaiming the 3.11TB and recreating the Fusion drive, am I out of luck here and just need to bite the bullet and go to an Apple Store?

Much appreciated.
I had the very same issue encountered with a iMac that has 1Tb Fusion drive.  22Gb was reported as SSD and then the rest of 1Tb of the drive.  After digging deeper into this problem, I noticed that the iMac Fusion drive was improperly configured and that I had to reformat it and create a Logical Volume, then configure the Fusion Drive to Apple Spec.

I found this article that helped me a bit, but I had to do a little improvising to get this procedure to work.

https://www.lifewire.com/setting-up-fusion-drive-mac-2260165