USB hard drive moved to internal, now shows GPT Protective Partition

Question

I'm using Windows 8.1. Using a brand new 1TB hard drive, I attached it to a SATA to USB connector, formatted it and transferred about 500GB of data to it. I don't remember if I formatted it as GPT, I assume that I did.

Now I've moved it into my Lenovo Y500's hard drive caddy to be used as a data drive. (This is the same laptop/OS used for the USB connection.) When I turned on the laptop I noticed that I didn't have a D:. I checked in Disk Management and found that the drive now says Healthy (GPT Protected Partition). Practically all of the right-click options are grayed out, including Change Drive Letter.

Side note: I have an SSD being used as the OS/boot drive which is assigned to C:. So this D: is only going to be used for data.

I checked in DISKPART and this is what it shows for Disk 1:

WDC WD10JPVX-22JC3T0
Disk ID: B5CAC984
Type   : SATA
Status : Online
Path   : 2
Target : 0
LUN ID : 0
Location Path : PCIROOT(0)#PCI(1F02)#ATA(C02T00L00)
Current Read-only State : No
Read-only  : No
Boot Disk  : No
Pagefile Disk  : No
Hibernation File Disk  : No
Crashdump Disk  : No
Clustered Disk  : No

There are no volumes.

I read online that I might be able to convert the partition from GPT to MBR to get rid of the problem without losing all my data. But I'm uncertain the steps that I need to take or if this is even the right path to clear us this problem.

I downloaded gdisk for Windows and this is what it says for Disk 1:

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: not present

Creating new GPT entries.
...stuff

Are you SURE you want to continue? (Y/N):

Should I proceed? What commands are needed after that to fix this issue and let Windows use the drive?

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Update

I tried using gdisk with Recovery then Convert GPT to MBR but this wiped the partition so all the data was deleted. As suggested, I backed up everything first by plugging the drive back into the USB (where the data was still accessible) and copying the files onto a network drive. If you try gdisk, definitely do the same if possible.

Answer 1

The problem was almost certainly caused by a USB enclosure that translated a disk's 512-byte logical sector size into 4096-byte logical sectors. This is explained in more detail here. If you're lucky, you might be able to restore the partition table by using gdisk on the disk attached in its original way. See the gdisk documentation on GPT repairs for general procedures. You'd need to read the backup partition table from the disk and hope that it's still intact.

Answer 2

After quite a bit of digging I stumbled on a solution. Using Disk Management set the Disk to Offline then Online and it will be accessible again.

Right-click on the Disk, not the Volume, where it says:

Disk 1
Basic
931.51 GB
Online

After right-clicking, select Offline from the context menu. The disk will how look like this:

Disk 1
Basic
931.51 GB
Offline (i)

Once again, right-click and select Online. The volume for the disk will now be listed as Healthy (Primary Partition).

This happened to work in my case, but it won't work with every volume stuck in GPT Protective Partition. Since it is so simple and shouldn't do any damage, it is worth a shot.