1. Storage and media for FCPX
Its important to start with the basics, and here are a few basics : store your media on an external drive that is fast enough to support video editing and streaming. As your media can be stored anywhere, don’t fill up your system drive with unnecessary clutter as it will slow things down when you’re editing (remember to try and keep your system drive around 10% empty or even 20% if possible so things run smoothly).

As we’re now well into the 21st century, use the fastest external drives you can get your hands on. USB2 and FW400 are not fast enough for todays practices, go for at least FW800 and USB3 or, even better, Thunderbolt if your Mac has the capability.
If you’re serious about editing, get a RAID set up to protect yourself against one of your drives dying on you mid-edit. RAID drives are faster and usually hold more data which is useful if you’re editing (remember that a single uncompressed 4K video frame is around 230mb at 10-bit depth!). Consider a RAID 5 array with a minimum of four hard drives as you’ll be able to edit pretty much in every format available (except 4K uncompressed).