I have a Mac Mini which I only use as a iTunes server. I have USB hard drives connected to it which house several TB's of MP4's. The hard drives are formatted as Mac OS Extended. I'm fairly new to Mac and always followed the rule of thumb in Windows to leave 10% of the drive unused for defragmenting. Is true with Mac formatted hard drives too? How much if any free space should I leave if the drive is just holding MP4 files for my iTunes server to host to my Apple TV's, iPad's, and iPhones? With 4.5 TB, i'm loosing close to 450gb of storage space leaving 10% free. Thanks in advance.