On linux machines you can fill your root partition to 100% utilization and the system would still continue to function giving you a chance to free up space without downtime, but on Windows machines the same is not possible and if you have managed to fill your C:\ to 100% then the system will go down (Know this for sure for Windows versions up to Vista, not sure about Win7) hard. This is something that has puzzled me a lot over the years but didn’t know how it worked, but now I finally have an explanation:
When you format a partition in Linux , 5% of the total space gets reserved for privileged processes, by default. This is done so that system processes continue to function correctly ,if the filesystem gets full. This is useful for your ‘root’ partition.
The really interesting part is that you can configure this to reduce the percentage of the reserved space using the tune2fs command. e.g. if you wanted to reduce the reserved space for /dev/sda1 to 1% of the total space then issue the following command as root:
tune2fs -m 1 /dev/sda1
Source: Flossstuff’s Blog