Windows apps do not touch the hard drive directly, ever, unless they're disk management software like Partition Magic. That's what the Win32 API is for. And SS2 uses an obsolete version of the Win32 free disk space call that doesn't understand disk sizes larger than 2GB.

I think it basically returns the free space modulus 2gb.
So, saving or downloading an extra 50mb of file(s) should make the free space 'wrap' back around and be detected correctly.

Example:
2050mb % 2048mb = 2mb of free space detected by SS2
2000mb % 2048mb = 2000mb of free space detected by SS2.

(For those who don't know, modulus (for which I've used %) yields the remainder of an integer division.)