Windows – How to format an HDD to GPT partition style using Windows 10’s installation disk

Did you happen to use Apple's Disk Utility to create a FAT filesystem in that to-be-Windows partition? If so, you converted the disk from a legal GPT disk into a hybrid MBR disk, which OS X sees as GPT and Windows sees as MBR. The solution in this case is to clear the hybrid MBR data. A number of utilities can do this. I'll describe how to do it with my own GPT fdisk (gdisk) utility:

1. Download GPT fdisk from its Sourceforge page and install it. (Versions are available for Linux, OS X, and Windows. I'll assume you'll do this from OS X.) Alternatively, you could run it from a Linux emergency disc like Parted Magic.
2. Launch gdisk on your disk by typing sudo gdisk /dev/disk1 in a Terminal window. (Change the device identifier if it's not as you presented earlier or if you use another OS for the job.)
3. Type p to view the partition table to verify you're working on the correct disk. If not, type q to quit without saving your changes and try again with another device.
4. Type x to enter the experts' menu.
5. Type n to create a fresh protective MBR. Note that gdisk won't confirm a change; it'll just show you a new experts' prompt.
6. Type w to save your changes. You'll be asked to confirm this action. Do so.

With any luck this will fix the problem. If it doesn't, though, you can use gdisk's v option (on any menu) to have gdisk look for partition table problems. It can fix some minor problems automatically, but other times you'll need to make explicit changes. See the GPT fdisk documentation on GPT repairs for details.

El Capitan Addendum:

Apple's OS X 10.11 ("El Capitan") includes a new feature, known as "rootless" or "System Integrity Protection," which makes it difficult or impossible to run certain types of utilities, including GPT fdisk. To be sure, this new feature is intended to improve security by making it harder for malware to take control of the computer or for users to accidentally damage their own systems; but it also causes problems if you really need to use GPT fdisk or other third-party tools. If Apple's own tools permit removing a hybrid MBR, I don't know how it's done with them, so AFAIK this task really does require bypassing this new security feature.

There are several ways to work around this problem, such as:

• Use a non-OS X OS, such as an Ubuntu installer booted into its "try before installing" mode.
• Hold down Command+R as you boot your Mac to launch the Recovery environment, in which this feature is disabled. In theory, you should be able to run gdisk in this environment, although you might need to adjust the PATH environment variable. (I've not tried this approach, so there may be hurdles I've not considered.)
• Boot to the Recovery environment, open a Terminal window, type csrutil disable, and reboot into your regular environment. This action disables the rootless system. You can re-enable it by performing these steps but pass enable rather than disable to csrutil.

For more on the subject of the rootless environment, see this page.

It looks like something has trashed your GPT data -- both the main data structures and the backup data structures. I don't know specifically what might have done this, but a buggy partitioning tool seems like the most likely culprit. Another possibility is that there's been some confusion over something like a RAID configuration -- if the disk was originally prepared with your motherboard's RAID features active and then accessed with the RAID features inactive, something like this might have happened.

Two options seem most reasonable at this point:

1. Re-create your partition "blind." The vast majority of modern partitioning tools begin partitions at sector 2048, so you could use gdisk to create a partition that begins at that point and that ends at the last possible sector. This stands a good chance of succeeding, but there's a risk that if your original partition did not start at sector 2048, you could end up damaging it when you try to access the new partition. Also, some tools will try to create a new filesystem when you create a new partition, so using them would be risky at best. (I recommended gdisk for this task because it does not touch the partitions' contents, just the partition table that defines them.) Also, if the disk held an EFI System Partition in addition to the main data partition, the main data partition would not begin at sector 2048.
2. Use a tool like TestDisk (included on many Linux emergency systems, such as PartedMagic and System Rescue CD) to search for your missing partition. (There are Windows equivalents to TestDisk, but I don't know what they are, offhand. You could do a Web search to find them, if you prefer to work from Windows rather than boot a Linux emergency disc.) This will be more likely to find your partition if it was placed strangely, but tools like this can become confused if the disk has been re-partitioned in the past -- they can sometimes detect the remnants of deleted partitions and try to re-create them even though they're no longer valid.

Take your pick as to the approach, and good luck! Whatever you do, though, be cautious. You might consider doing a raw disk backup to a spare disk, if you've got one that's big enough to hold all your data. If you do this, though, be very careful in specifying the source and destination disks, or your backup could end up trashing the data you intend to save!