I saw a presentation years ago that said hard drives had the best performance when they were < 50% full and that for busy servers, you want to keep your drives < 80% used. The reasoning was that the tracks are written from the inside out and that access, especially random access, was quicker for inner tracks than outer ones. Rotational latency was lower.
On the other side, with today's caching, and sometimes read ahead in products like SQL Server, a longer outside track, with no track to track movement, might be negating factors.
Is this true? Is there a reason to keep space free on a modern hard disk system? Is it different for Windows than *Nix?