It's probably safe to say you can remove that unknown from permissions but it's probably safe to say you could also leave it.
As all you want are files, I assume you will be deleting all partitions afterwards and using the drive as a healthy secondary..
Try adding Authenticated Users? See if it changes anything.
- Right-click on D: drive and click Properties.
- Click on the Security tab.
- Click Advanced.
- Click the "Change Permissions" button.
- Click "Add"
- Click "Select a principal".
- Type "Authenticated Users" and click OK.
- Select "Modify", and click OK.
- Click Apply, and click OK.
(focus on the steps more than the images, all I could find on short notice)
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Full control below would be fine.
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