Do you fill a secondary hard drive to full capacity?

Leave 20% free space on your OS drive if you use hibernation and don't empty the recycle bin very often. If you use don't use defrag tools, use the "Always On" power profile, and use Shift key modifier to bypass the recycle bin on deletion for most items, and disable unnecessary system services, you shouldn't have any large dynamic system files that shrink and grow with usage. You can then get by with 10% free with System Restore set to take 5%. Try not to store your files (docs, downloads, pics, etc.) on the OS partition. This will keep the files sizes generally small.

I leave System Restore on for the OS drive because it may be useful in restoring system files and settings that have become corrupted. It is not a solution for backing up a whole partition's worth of files, because it is limited in what it monitors and restores.

I turn System Restore OFF for secondary drives I'm using for static storage.

If you use a disk defragging or optimizing tool, I would leave 20% free. This is so your defragging and optimizing programs have room to work while shuffling around files on a disk. Windows defrag, which has traditionally been based on Diskkeeper, requires 15% free space to work efficiently. I leave an extra 5% as a cushion. If the drive is large (say > 500GB), go with less. If file sizes are huge (say ~ 50GB), as in the case of stored partition images, then go with more.

The one that comes with windows requires 15% to run efficiently. However Windows Defrag, based on Diskkeeper, is limited in it's usefulness. It does a piss-poor job of consolidating free space after making your files contiguous. the end result is that you have file fragmentation the next time a moderately large file is written over the scattered small free space fragments. Also, NTFS does a pretty good job of reading and writing files to a fragmented drive, anyway. Your choice, there are others out there, if you like to defrag.

Storage is cheap, though, so you can be safe and raise it too. It's just good practice not to be near the ceiling. That way your computer wont' balk when you accidentally drag that HD movie recording to the wrong drive. I don't like to be under 10% free, in any case.

Hope this helps.

Zach