External HD not appearing in scratch disc menu in Photoshop 2020 on Mac

2TB Seagate Backup Plus Slim isn't showing up in the scratch disc menu, only my computer's local storage. I formatted the disc for Mac when I got it, so that's not the problem. I ran it through Disk Utility first aid, that's not the problem. In other Adobe products such as After Effects and Premiere, I've had no trouble with storing cache, auto-saves, and more, though I understand that there are some fundamental differences between that and a scratch disc. I've attached a video showing my problem and the basics of the HD. 

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