An Issue Comes Out When Converting the Drive to GPT to Use All of the Space, Any Help?

I have a 5 TB drive installed for gaming, and it's currently set as MBR and denying me half my space. The option to convert to GPT is grayed out. Windows 10 is installed on the original drive, and I'm seeing another System Reserved partition on the 5 TB drive. The 5 TB drive is just for running games, so there isn't any data I'd miss if I had to wipe it. I'm not super confident in what I'm doing, so I need some guidance.

As you said, you have a 5 TB drive and can't use all of its space since it was set as MBR. And the most distinct advantage of a GPT disk is it can support more than 2 TB storage capacity, which MBR can not. Therefore, the best way is to fully convert the disk from MBR to GPT to use your 5 TB disk space. 

In addition, the reason why "Convert to GPT greyed out" is that Disk Management can only convert a disk to GPT when it is empty. So you have to delete all of the partitions on the disk before the conversion. So my suggestion is to back up your essential data in advance. Here is the step-by-step tutorial:

• Delete Partition: Right-click any partitions on the target disk and choose "Delete Volume. (Click "Yes" if a window pops up and asks do you confirm to delete all data on the partitions will be deleted.)
• Converting to GPT: When the entire disk show as an unallocated space and the greyed-out problem is gone. Right-click it and select "Convert to GPT Disk." 

If the first solution is a bit complicated, you can resort to EaseUS Partition Master, a one-for-all partition optimization tool. It can easily convert from MBR to GPT without data loss, as well as convert from GPT to MBR.

Check the steps of how to use EaseUS Partition Master:

Important: For data security, you'd better restore the data to another storage device for recovery.

This software has many other features, you can also use it to extend system partition, format a partition, and merge two partitions into one.