Why? Is the product I really need available off the shelf? Consider the need in both technical and financial terms.
How much? Is there sufficient volume to justify the development of a custom product?
When? How long is my design cycle? When is the drive really needed for machine integration?
How? How is the customization program going to be managed?
What? Expend the time and effort to properly and completely define the product. Get the vendor to write the specifications and ensure they understand what you need.
Who? Determine the companies that have the technical capabilities required and the business model to make a custom product.