Reclaim lost space from Bootcamp Partition First off, sorry I didn't understand you had to use apples bootcamp software to deal with bootcamp partitions. I wanted to stop using bootcamp, so I deleted the partition through disk manager some time ago. Since then I have been trying to get the 400GB space back. 

Today I followed this thread:
Cannot delete or merge old boot camp partition in El Capitan

And I deleted the partition that showed up. But the result is that it has simply vanished, I can't reclaim it, because it does not even show up on disk manager or diskutil.

I have the 3TB fusion drive, but only 2.7 GB is available. I'm on a late '12 imac with high sierra 10.13.3.









===== final result ======
I suggest creating the USB flash drive installer, because it takes you 30+ minutes to boot to Internet Recovery. If you want to use Internet Recovery then use the alternate instructions given below.


Backup Macintosh HD with Time Machine to an external drive.
Download High Sierra from the Apps Store. If the download software starts to execute, cancel the execution.
Transfer High Sierra to a USB flash drive. See How to create a bootable installer for macOS.
Restart the Mac and immediately hold down the option key. When the Startup Manager icons appear, choose the installer.
From the menu bar, choose the Terminal application.
Enter the commands given below to create the new Logical Volume Group (LVG)

diskutil  unmount  "Macintosh HD"
diskutil  cs  createLVG  "Macintosh HD"  disk0  disk1

Use the command below to determine the UUID of the new LVG.

diskutil  cs  list


For example, you would look for a line similar to what is shown below.

+-- Logical Volume Group A8CEEF75-6839-4806-B799-97CF0CAABB7C

Enter the command given below to create a new Logical Volume (LV).  

diskutil  coreStorage  createVolume  &lt;UUID from previous step&gt;  jhfs+ "Macintosh HD"  100%


For example, drawing on the example in the previous step, you should enter a command similar to what is given below.

diskutil  coreStorage  createVolume  A8CEEF75-6839-4806-B799-97CF0CAABB7C  jhfs+  "Macintosh HD"  100%



  Note: Replace A8CEEF75-6839-4806-B799-97CF0CAABB7C with the UUID you determined in the previous step.

Quit the Terminal and use Time Machine to restore your files.


  Note: The OP has reported a complete verbatim restore works better than selectively restoring files.

Install High Sierra. Be sure to select that you what to keep your files.


Alternate Instructions


  Note: Requires macOS Sierra 10.12.4 or later,.


There instruction require booting to Internet Recovery.


Backup Macintosh HD with Time Machine to an external drive.
Boot to Internet Recovery. At startup or restart, immediately hold down the option+⌘+R key combination.
From the menu bar, choose the Utilities-&gt;Terminal.
Enter the commands given below to create the new Logical Volume Group (LVG)

diskutil  unmount  "Macintosh HD"
diskutil  cs  createLVG  "Macintosh HD"  disk0  disk1

Use the command below to determine the UUID of the new LVG.

diskutil  cs  list


For example, you would look for a line similar to what is shown below.

+-- Logical Volume Group A8CEEF75-6839-4806-B799-97CF0CAABB7C

Enter the command given below to create a new Logical Volume (LV).  

diskutil  coreStorage  createVolume  &lt;UUID from previous step&gt;  jhfs+ "Macintosh HD"  100%


For example, drawing on the example in the previous step, you should enter a command similar to what is given below.

diskutil  coreStorage  createVolume  A8CEEF75-6839-4806-B799-97CF0CAABB7C  jhfs+  "Macintosh HD"  100%



  Note: Replace A8CEEF75-6839-4806-B799-97CF0CAABB7C with the UUID you determined in the previous step.

Quit the Terminal and use Time Machine to restore your files.


  Note: The OP has reported a complete verbatim restore works better than selectively restoring files.

Install High Sierra. Be sure to select that you what to keep your files.