A week ago i noticed that the back corner (where side meets cuticle) of my big toenail had turned white, now it is starting to ache and is separating ?

Infection: It sounds like you have developed an infection in the area. See a podiatrist for treatment.

Consider spur.: It is more than likely the beginning of a nail infection. Having said that, it could also be caused by a bone spur, off the toe bone. As the spur grows, it can push the nail upward causing similar symptoms. Though not as common as a nail infection, it might be wise to have a toe x-ray, helping to remove all doubt.