Cannot select a new scratch disc

[defaultfhnea32t97a4]

I have had the issue of my "scratch disc" being full whenever I am on or trying to launch Photoshop for months now. It has gotten to the point that I cannot even open now. I have read tons of similar posts about this problem and it doesn't seem like it gets solved. Adobe/Photoshop will NOT let me select a new scratch disc. I have tried every possible suggestion on the internet. I use a Macbook Pro and I have a large WD external hard dive I specifically got in hopes that I could use that for the new scratch disc location. Someone please help! I honestly don't know what else to do except buy a new laptop or computer..... -__-

[c.pfaffenbichler]

»Adobe/Photoshop will NOT let me select a new scratch disc.«

Does the disc appear on the list or not? 

https://helpx.adobe.com/photoshop/using/scratch-disks-preferences.html