Specifics for the auto-defragmentation: With HFS+ on OS X, whenever the operating system accesses a fragmented file under 20 MB, it checks to see if there is a

Specifics for the auto-defragmentation: With HFS+ on OS X, whenever the operating system accesses a fragmented file under 20 MB, it checks to see if there is a large enough contiguous block of free space elsewhere on the disk to defragment it to; if there is, it silently moves it. Thus, you're almost guaranteed to never have fragmented files under 20MB (unless your disk is a complete mess). Above 20MB, you typically don't get significant fragmentation until your drive is nearly full and the system has to pick and choose free space blocks from all over.