The Samsung Galaxy S i9000M (aka the Vibrant) from Bell Mobility seems to be having a high rate of failure with the internal SD storage. Starting back in September, some folks started seeing their internal SD card become unreadable, unmountable, or just plain invisible to the kernel. There seems to be a few things in common here -- most of the affected users are running Froyo, most of them have also applied some flavor of "lag fix," and the only sure-fire way to fix it is to ship the phone back to Samsung.
There's a lot of theories, ranging from bad hardware, inferior components, and unsafe hacks. After reading for half the day, I think it's a combination of all three. It certainly seems like folks who have upgraded to Froyo but haven't mucked around with their file system aren't seeing many issues, and those that have used some sort of lag fix are seeing the most. Of course Samsung didn't use hardware that did not meet their specs, but they also didn't use the absolute best available -- that's not very kind to the bottom line. SD media doesn't take repartitioning very well, and it's likely that the hours spent applying and removing various lag fix schemes has caused early failure for many. Add in the ever-present bad piece of hardware, and you have the makings of a mess. And since the only real fix is to send it in and spend time without your phone, my advice is to stop messing with the file system -- inflated Quadrant scores just aren't worth it. Thanks Gregg for bringing this to our attention!