A quick search shows that GPT is a protected storage system. Initially I took this as meaning that the data was encrypted but then it occurred to me that the protection goes deeper than that which is why you were not able to totally reformat the drive, only the free space. Something with that portion of the drive simply will not allow access and I don't know enough about system formats to say what that is or how it works.
I found your question posted at a Windows help site and briefly read some of the replies. It doesn't appear any of the replies offer a solution but here is the link if you would like to dig in deeper.
Some of the replies I skimmed talked about the original enclosure. I'm wondering if they were hinting at the same thing we do when we want to share encrypted data on a phone or tablet, where you decrypt the data with the original device, using the original device to transfer the data. In fact if I were to offer advice that would be my recommendation. Does the device have ports that could be connected to something external? Again if it doesn't that may be intentional to prevent these types of actions. You may also find that the encryption is one that can only be decrypted by that specific device further limiting unauthorized access or transfer.
The key to this seems to be the question, did they use this format intentionally to prevent the data, movies and shows I'm assuming, from being shared outside of the control of their TOS and EUA? It may just not be possible or at minimum so ridiculously difficult that it deters such efforts. I stopped at that the first, very first in-fact, search result that matched your question about 95% so if you don't want to accept this answer you can try your own search and see what results you find.
Let us know what you find.
Best wishes.
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