My iMac hard drive has been halved

Notice the large chunk after GPT3 on disk1. We are going to create a 'fake' GPT entry and make it look like a FAT32 partition, that the BC Assistant can then Remove/Restore. Please post any error messages that you encounter during these steps.

First, we need to disable System Integrity Protection using the following steps.

You have either El Capitan or Sierra. You will need to disable SIP by booting into Local Recovery (using Command+R - not Internet Recovery) and click on Utilities -> Terminal and type

csrutil status

csrutil disable

csrutil status

Boot normally and check the output of csrutil status which should now be disabled.

Second, we will create a new GPT4 using the following steps, after GDisk (aka GPT Fdisk) is installed (do not enter text like this. It is for informational purposes).

sudo gdisk /dev/disk1

x (Experts menu)

l (set sector alignment to... this is lowercase 'L')

1 (... to one sector)

m (Main menu)

n (New)

4 (GPT4)

865634440 (Start sector)

+1087890695 (Size. Notice the '+' sign)

0700 (Microsoft Basic Data)

p (Print the GPT)

w (Write the GPT)

y (Confirm write)

q (quit, if necessary)

Third, we need to repair disk1 so the disk 'knows' that we have a GPT4.

diskutil repairDisk disk1

If everything works correctly, you can now run BC Assistant, and click on Restore/Remove.

Once you get your disk space back, re-enable SIP by using Local Recovery and

csrutil enable

I suggest you back up OS X, before your next adventure in Windows Wonderland.

Sorry for the delay in getting back to you. I was having diner.

First thing to do, is have a good backup in place just incase something goes wrong or you can not return the whole drive back to one partition. Time Machine will work in a pinch, but it is slower than using a cloner like CarbonCopyCloner or SuperDuper.

Now for the second question, what does the Device Information say when you click on the solid blue portion of the disk?

Hmmm.... i was hoping that it would give us some information and that the - button would become available so that you could remove that portion and add it back to the main partition.

It generally works without destroying the first partition, but in this case to be honest, I'm not sure if it was the removal of Bootcamp or if it is the Fusion setup that is blocking the removal.

(Most likely something went wrong while removing the Bootcamp partition)

Once again, a good backup is important just incase you end up blowing out everything and starting over.

Personally I think you need to backup, erase, reinstall and restore. I suspect it should be possible to resize the free space that was used for bootcamp but I'm not totally sure how core storage & bootcamp work together.

I'm tempted to ask for this to be moved to the bootcamp forums you may get more help there - any opinion on that?

From Applications -> Utilities -> OS X Terminal, please post the output of

diskutil list

diskutil cs list

BC Assistant is supposed to merge freed-up space back into the CS LV/LVG that is used to manage the Fusion drive, but there are numerous failures, and the newer macOS/BCA versions are somehow worse than older versions at such tasks.

This should be correctable. You will need GPT Fdisk (https://sourceforge.net/projects/gptfdisk/).

Post the output of

sudo gpt -vv -r show /dev/disk0

This will prompt you for your password, which will not be echoed back to you. This will show the GPT entries. You should have a large chunk of free space on your 1TB disk drive.

Select the Fusion Drive, click on the Partition tab and tell or show us what you see.