You should aim to keep a minimum of 10% of your startup disk’s capacity free so OS X can perform various housekeeping tasks. Any less than this and you’ll suffer a serious
performance hit. Replacing the internal drive can be impossible (again, this isn’t a serviceable part on the latest MacBook) or difficult (on the late 2012 Mac mini it’s just under the top surface, which means, because you access the innards from below, you need to remove everything to reach it – entrust this task to a professional). If you’re using an older MacBook or iMac with a hard disk, this may be spinning at 5,400rpm. Upgrading to a 7,200rpm model is a sensible, economical move, but if you can afford the next step – a solid-state drive (SSD) – you’ll really notice the difference it makes.