steam is terrible when it comes to this sort of thing.
Losing downloaded data sounds more like a hardware failure more than a fault on Steams part.The only time I've had data go randomly missing was when I had a virus that randomly deleted files a long time ago
losing 2gb;s of downloaded stuff? ._.
steam is really annoying in the way that it downloads and updates. i've lost l4d2 before, all i had to do was redownload it.