Boot camp partition option is greyed out

Is it a standard 3TB HD or a 3TB Fusion drive? Please post the output of the following OS X Terminal commands.

diskutil list

diskutil cs list

From the Terminal, do you get any errors, if you run

diskuil repairDisk disk0

disk0s1 is your EFI partition. The question is to ensure you have a backup. If the EFI partition is corrupted, diskutil will re-create the EFI partition. I have seen only one occasion of an actual erase, because someone had tried to store their own version of EFI.

Continue with a 'y'.

If you already have the USB installer created and are installing Windows 7, the please see Install Windows 7 and earlier on your Mac using Boot Camp - Apple Support (specifically step 11) to updated your USB Installer manually. If you have already done this, you need to use only the last checkbox - "Install or Remove Windows...".

Can you disconnect all external storage and run the following two procedures and then re-try BC Assistant and partitioning?

Reset the System Management Controller (SMC) on your Mac - Apple Support

How to Reset NVRAM on your Mac - Apple Support

In Disk Utility, select the entire disk. It should already have OS X partition.

1. Click on the partition drop down and make it three partitions.

2. Create the second partition as a FAT32 partition. Please ensure you adjust the OS X partition to an appropriate size such that this partition starts within the first 2TB of the disk. You can also try to split the whole disk in half, and use the upper half to further split it into your main OS X and a FAT32 partition of the size you want Windows to be.

3. Create a third partition which is Mac OS X Extended (Journaled). It should match the type as the main OS X partition. This will be combined with the first OS X part to form a CS LVG/LV to make it look like a single OS X logical disk.

Please post the output of the following OS X Applications -> Utilities -> Terminal commands

diskutil list

diskutil cs list

sudo fdisk /dev/disk0

The last command will prompt you for your password, which will not be echoed back to you.

1. Download GPT Fdisk from https://sourceforge.net/projects/gptfdisk/. This will help create a MBR.

2. Please post the output of

sudo gpt -vv -r show /dev/disk0

After Gdisk is installed, rebuild MBR to match the new GPT information thus resetting the Hybrid MBR. Use defaults for other questions (like partition codes). The only values that need modifications are the Boot flags and step 10. Accept all other defaults that Gdisk offers. Please see thesample Q&A as an example. These steps can be repeated if you make a mistake before you get to Step 12, otherwise start from Step 1 for these steps. Step 6 has numbers which are typed with a space between the numbers. Please see the sample Q&A before you execute these steps. We use only 2 3 4 because GPT 5 is not necessary to be included in the MBR.

1. Sudo gdisk /dev/rdisk0
2. P (Print list of parts)
3. R (Recover)
4. O (print current Hybrid MBR)
5. H (chooses Hybrid)
6. Partitions numbers to be hybridized: 2 3 4
7. Y (Good for GRUB question)
8. N (part 2 boot flag)
9. N (part 3 boot flag)
10. Y (part 4 boot flag make NTFS bootable partition)
11. O (print current Hybrid MBR)
12. W (Write the new MBR)
13. Y (Yes! write the new MBR)
14. Reboot

Here is sample Q&A for this section. Please notice the Press Enter/Return.

Place EFI GPT (0xEE) partition first in MBR (good for GRUB)? (Y/N): Y

Creating entry for GPT partition #2 (MBR partition #2)

Enter an MBR hex code (default AF): Press Enter/Return

Set the bootable flag? (Y/N): N

Creating entry for GPT partition #3 (MBR partition #3)

Enter an MBR hex code (default AB): Press Enter/Return

Set the bootable flag? (Y/N): N

Creating entry for GPT partition #4 (MBR partition #4)

Enter an MBR hex code (default 07): Press Enter/Return

Set the bootable flag? (Y/N): Y

Please reboot, use only the last option (Install...). If you see only (Remove...) please post back. Since Bootcamp is a FAT32 partition, please follow the steps as documented in https://help.apple.com/bootcamp/mac/5.0/help/# for formatting this FAT partition.