How to get the location of the first byte of the file on a disk?

I need to checksum every single file on a given USB disk in a C# application. I suspect the bottleneck here is the actual read off the disk so I'm looking to make this as fast as possible.

I suspect this would be much quicker if I could read the files on the disk sequentially, in the actual order they appear on the disk (assuming the drive is not fragmented).

How can I find this information for each file from it's standard path? i.e. given a file at "F:\MyFile.txt", how can I find the start location of this file on the disk?

I'm running a C# application in Windows.