hard drive storage

I have an XPs 420 which is about 3 years old.  It has 283GB of memory, and I have it almost full.  I have performed disc defragmentation and file cleaning, but it has little effect.  Dell diagnostics shows that the system is "performing normally", and a scan shows that everything is healthy.  When I use the drive space manager to get the pie graph representation, it show that most of my memory is "unscanned".  I do digital graphics and animation, so I do use up a lot of memory, but I store most of it on an external hard drive.  I have deleted everything I can to make more room, but most of my memory is still unaccounted for.

Help!