Complex Mesh to Surface in Rhino

Hi there,

I have a complex 3D form which I have created in Grasshopper through Target Boxes, taking a basic minimal surface geometry and target boxing it around a grid to form the shape. 

I am relatively new to Rhino, but I find myself struggling to be able to do anything with it (particularly as a mesh). I thought perhaps turning it into a NURB surface would give me more joy but I am finding it impossible to do so. 

As a mesh, I played with the control points to create a more desired shape that I needed after baking it from Grasshopper. The MeshtoNURBs command refuses to do anything. T-Splines also just warns me with me 'T-Splines has crashed, please save and restart Rhino'. 

I mean I may not need to turn it into a polysurface but I find taking elements off the geometry for further modelling needs it to be a polysurface, I may be wrong. But I presume it would be still useful. 

I can't even upload a fraction of the model as it is too big. (Could this be the problem, and if so - how to solve it?)

Thanks for your help!