Can I use an external SSD as a scratch disk in Photoshop?

I am on a Mac. I was under the impression that I could use an external SSD as a scratch disk in Photoshop. I purchased a 1TB My Passport Go. It is not showing up in Photoshopscratch disk preferences as an option. I reached out to Adobe support and they said that I can only use my internal storage as a scratch disk. I have 17GB of free space left and I am working with very large print files that are constantly filling up my scratch disk. I have cleared as much as I can from the scratch disk to make room for storage. What do I need to do?