Farran De Tao / Lĩ PhongPhotoshop is such a lil twerp. Even with 9GB free space, it refuses to run because "the scratch disks are full". Well I even attached a second SSD with 70GB free but it didn't accept that (despite it working fine last week)
The solution was to run file compression on the the Windows OS files and on most of the programme files. I reclaimed about 7GB, and finally Photoshop would launch.