I've done this with 4TB drives and going backwards it starts reading in a matter of seconds. Maybe you're writing to a filesystem that doesn't support sparse files like a network share? Sparse files eliminate any need to pre-allocate all the space. Well, you do pre-allocate, but instead of writing "0" ten million times, you just have to write "0x10,000,000". This is an over-simplification but you get the idea.