My primary drive is for my operating system... I have other (much larger) drives for my programs, files, etc... and one for cache / scratch space. Working on a large document in both Designer and Photo, it's sucking up three gigabytes on my primary drive because I can't assign that temp space to a drive where I have terabytes of space. Like I can in Photoshop and Illustrator!
I really love the Affinity suite, but I'm probably going to have to move back to Adobe products for certain things, in part because of the lack of an assignable temp / scratch drive.
Please add this option! Folks have been asking for a while now, apparently...