How do you save using system?

In my game I need a way of saving right about 263169 floating point numbers fairly quickly, and I have found out that player prefs will not cut it. My main question here is how can I use system and do something like create a single file that I can read and write to? Or use system somehow to save all of my data?
Thanks!

Try this question for XML writing and this one for ini writing.

But out of curiosity, what are you doing?