Sorry, but you're yet another person on the internet making false assumptions, and starting rumours about technologies that will worried grandmas and amateurs forever.
The camera knows whether it can store the shot or not before it lets you take it. It knows the max file size of a picture, and will let you know if there isn't room on the card for that by saying "0".
Actually... I sadly speak from experience on the corrupting of the contents of a 2 week old brand new CF card. I did not mention it because it is rather embarrassing. I had to do not a quick format, but a full format to make it useable again, the recovery software that came with my did nothing for me. The card was fine after a full format.
And no, a camera (at least not my D200) cannot tell you in advance how big a picture will be, therefore, how can it know if it will fit that last one or not?
My NEFs range from (on one shoot to another ) from 14.9MB to 16.4MB depending on several factors but me not changing the file size, compression or quality settings. The camera will try to average it but it cannot know in advance how big that picture will be. I filled an 8gb card and it failed on my last pic... no pics were ultra important as I was playing downtown, so I had no issues with the lost pics, but it was still a PITA and a lesson to learn.
This is no false assumption or rumor... this is a true fact and it happened to me. I leave the choice for all to do as they wish, we are all big boys and gals here.
And discussing about hard drives, if one fills a system volume on most Windows operating systems from Windows 95 to Vista to very close near capacity and the page file is not set at a fixed size, it will not slow down at that point much at all (it does slow down when your physical ram runs out and tries to replace it with virtual ram AKA your swap or page file), you will get a low HD space warning and if you ignore that, as the swap file tries to expand to accomodate the needed extra virtual ram, and fails, the operating system will then sometimes (but not always) give you a nice BSOD. If the BSOD happens, you have about a 50% chance of having corrupted your hard drive.
A lot actually depends on what program your called that needed the additional RAM and how it responds to the OS telling it there is not enough memory to run it. A poorly written application will try to force a write to a portion of memory already used by the OS, and thats where you get your BSOD.
As far as a secondary drive or partition, you can fill that to 100% capacity without concerns as long as you do not place a pagefile or use it as a location for temporary files that the OS needs.