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?

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?

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.