Well, I found it this morning in the book, A Mind At A Time by Mel Levine, MD:
Fine motor and graphomotor function are quite different. Graphomotor function is the highly specialized motor output used in writing. Many students boast superb fine motor abilities and unacceptable graphomotor function... [a young boy with poor handwriting] was a born graphic artist. From the age of two, Raoul loved to draw. At nine his cartoon creations displayed precise fine motor control. But somehow Raoul could not engage in the rapid assignment and activation of his finger muscles required for letter formation, a classic example of strong fine motor function accompanied by a stubborn graphomotor dysfunction.
Some of the reasons Dr. Levine gives for graphomotor dysfunction are:
1. Feeble connection between his memory and his fingers ("A very heavy flow of memory takes place when your child sets out to put things down on paper...that child needs to be able to recall letter shapes and the chain of tiny muscle movements he needs to execute them.")
2. Motor implementation problems ("Certain muscles are brought into play to grasp a pen or pencil with reasonable firmness, while other finger muscles are supposed to keep it moving in the desired direction... For some children this very precise assignment and mobilization of specific muscle groups on demand is all but impossible... He may exert far too much pressure, write with a fistlike grasp, or maintain his pen perpendicular to the writing surface."
3. Finger agnosia ("The need for feedback during writing never slackens. You have to know just where your pen or pencil is at all moments while you're etching those letters. Some students harbor a condition we call "finger agnosia." Believe it or not, they lose track of where their fingers are. When still quite young, they keep their eyes close to the page watching those digits diligently, substituting visual feedback for data that should be coming from the joints and muscles of the fingers themselves.")
Dr. Levine recommends that children with consistent struggles in penmanship be allowed to use word processing programs to write out their schoolwork (we do this at our house). But he also recommends that they keep plugging away at copywork: "...they also need consistent practice forming letters. Many require help developing a more workable way to hold a pen or pencil."
And my own two cents: With these kinds of issues, it is especially important to assign copywork geared to the child's skill level and not his or her "grade level". We want to challenge without frustrating.