(or why does Windows complain of low disk space when there is enough available?)
This was an issue I was running into, trying to copy a large 17GB file onto my portable, external hard drive. It had about 60 gigs free, and yet, everytime I tried to copy the file, Windows would complain about lack of disk space. It was frustrating.
Until I realized, that the external drive was formatted with FAT, which has a 4GB limit on file sizes. Really stupid error message, but what else could one expect from Windows? Anyway, changing that to NTFS fixed the problem.