Can’t uncheck or select new scratch disk

I have an older version of photoshop. I can see in preferences where I am supposed to be able to uncheck Mackintosh as the default start up scratch disk but I can't uncheck the bos or add a new scratch disk. Nor can I do it by holding down option/command key when photoshop starts up either. The window that pops up doesn't allow me to change scratch disk from Mackintosh internal hard drive p. I bought a brand new 1 terrabyte Samsung hard drive and was hoping to set this up as a faster better scratch disk but can't see any way to do this. The option doesn't appear. Have looked at previous discussions-no answer......