I just realized why this doesn't work. Magick.NET is build with HDRI support which means each channel uses 4 bytes. For each pixel this will be a total of 16 bytes (RGBA). This will be changed in ImageMagick 7, only 1 channel will be used for a grayscale image.
Your output image has a size of 31200x15152. That is a total of 7563878400 bytes, which is 7gb. In a 32 bit application the maximum about of memory that can be addressed is 2gb and that is why Magick.NET is using your hard disk to store the pixel data.
Did you run your test with the command line with the option: -define registry:temporary-path=d:\?
If possible I would advice you to switch to the x64 version of Magick.NET, this will allow you to address more memory.
Your output image has a size of 31200x15152. That is a total of 7563878400 bytes, which is 7gb. In a 32 bit application the maximum about of memory that can be addressed is 2gb and that is why Magick.NET is using your hard disk to store the pixel data.
Did you run your test with the command line with the option: -define registry:temporary-path=d:\?
If possible I would advice you to switch to the x64 version of Magick.NET, this will allow you to address more memory.