Comment by wccrawford

Even as a native, I didn't know the difference until I looked it up right now, and I'm almost 50.

Short form, for those who wonder:

"That" is used for things that are important, and "which" is used for things that aren't. In the first sentence, they want the blue floppy disk in particular. In the second, they want the floppy disk, and it happens to be blue. The "which" is added to aid the listener, not to because the color blue is important.

If that's interesting, it's worth Googling for a more precise answer.