Faces Partially disappear from mirrored object?

Hi, I ran into a new problem today- I duplicated an object and then mirrored it, but after it was mirrored the majority of the faces were gone?

Any Ideas what’s is going on here?

The object is not set up correctly.

Please post your .blend file.

I figured out that the issue was that the Normals needed to be flipped. Is there a way to prevent this from happening in the first place, or a way to correctly mirror it that prevents this?

If in doubt - hit Ctrl-n and w - Remove Doubles in edit mode from time to time.
Always click Ctrl-A - Scale, Rotation in object mode before trying any modifiers.

There is no way blender can figure inside/outside if you start modelling not using default objects. For the default plane you might be alone on what’s in what’s out.

Make sure blender knows that, which you did ;)…