Creating data drive out of Boot Camp

I installed W10 Pro x64(1511) on Boot Camp partition of Macbook(El Capitan). I am unable to create a data drive out of the Boot Camp drive. I am using a third party utility - iPartition 3.5.1 which is installed on OS X. When I attempted to shrink the Boot Camp partition(370GB) to 100GB, error message displayed said:
Repartioning Failed
iPartition was unable to get write access to the device
"APPLE SSD AP0512J", (disk0). This may mean the
device is write protected, that you do not have
permission to access it, or that it is in use by
something else.

I am wondering if anyone did what I am trying to do.

Not really enough information but here are some things learned from past forum posts regarding the same subject:

1. Are you trying to partition while the drive is mounted? If so, you can't do that without booting from a separate drive containing iPartition. I believe iPartition even tells you that in its instructions.

2. Repartitioning a boot camp partition (further partitioning it) will likely lead to destroying the Windows 10 boot sector.

3. There is no need to create a "common data drive" (if that's what you're doing). Just download, install, and use Paragon NTFS from your OS X partition as that will give you full access to all your Windows data from OS X.

And yes, many folks have tried what you've tried and wound up having to reinstall OS X and Windows. ;D

PS: Make sure you backup both your OS X partition and Windows partition before attempting anything else.

I think the problem is trying to resize the Bootcamp partition. Mac utilities generally don't handle that since the BootCamp partition is formatted NTFS. Perhaps a Windows based utility could do the job of you direct booted to Windows 10 but I'll have to leave the recommendation of a program to someone else. The other options which occurred to me might not let you create a data partition in addition to the Windows partition.

1. Delete the Windows partition using the BootCamp Assistant and re-create the partition at a different size. Then re-install Windows on the new partition. I'm not sure that would create a data partition as well.
2. Paragon Software has a utility called CampTune which allows you to readjust the Windows partition. That might also allow creation of a data partition. The page I linked to has the manual and might give you some ideas.

Edit: Once again Charlie for the score. A better answer than mine and faster. How do I top that?

chscag said:

1. Are you trying to partition while the drive is mounted? If so, you can't do that without booting from a separate drive containing iPartition. I believe iPartition even tells you that in its instructions.

Click to expand...

I have found that you are right. I created bootable USB flash drive, booted from it and found repartitioning could be done but aborted because a message said "Partion will be destructively resized"

2. Repartitioning a boot camp partition (further partitioning it) will likely lead to destroying the Windows 10 boot sector.

Click to expand...

This seems to be the case as above.

3. There is no need to create a "common data drive" (if that's what you're doing). Just download, install, and use Paragon NTFS from your OS X partition as that will give you full access to all your Windows data from OS X.

Click to expand...

I just do not want to place data files on the system partition.

And yes, many folks have tried what you've tried and wound up having to reinstall OS X and Windows. ;D

Click to expand...

But if anyone has luck doing it. ;D

How can I do what I want to do now? What about destructively repartitioning the Boot Cam partition to create two partitions and install W10 on one of them? Does this work?
Slydude mentioned CampTune which can repartition Boot Camp or shrink and create another partition. I reviewed info on CampTune website and found no mention of such damage.

Slydude said:

2. Paragon Software has a utility called CampTune which allows you to readjust the Windows partition. That might also allow creation of a data partition. The page I linked to has the manual and might give you some ideas.

Click to expand...

The CampTune seems to be able to change size of the existing two partitions or reduce one and increase another but not to create a third partition. I wonder if there is anyone who could add a partition by using CampTune or any other means.

@Charlie, Does the following get churin to what the wants to do? I'm not sure it works.

@churin Danger Will Robinson!!! Don't try the following without backups of both your Mac partition, Windows partition and any important data. (Slydude arms flailing wildly in best robot impression).

1. Backup both partitions. I'd suggest "cloning" the Mac partition so you can boot from it should things going go wrong.
2. Create the data partition out of part of the OS X partition. OS X will want to format the drive as HFS+ which you can read from Windows using the HFS+ reader from Paragon. It should also work if the partition is NTFS using the reader Charlie suggested.

Doing this might be impossible. I don't remember for sure if OS X will allow you to create the data partition out of the OS X partition. If this is possible, it might require deleting the Windows partition and basically start from scratch. That's why the need for the backup.

I am pleased to report that my problem is resolved by doing the way I knew 200% as a Windows user!!

Windows comes with a utility to manage disk partition. By using the utility I could shrink Window system partition and create a new partition. There is now OS X partition, Windows partition and a data partition. One may want to install a third party utility on OS X to read/write on the data drive.

It appears that I can forget Mac as long as I am using Windows on it.
So far I can switch between OS X El Capitan and Windows 10 Pro x64 just fine.