If all you're worried about is someone from the outside accessing your data, the "safest" is going to be keeping all your data on an external that can't be accessed through an IP.
If you're worried about HDD corruption, then keeping your data on a fileserver in a non RAID0 config is going to be the best bet.
If you're worried about losing files, back everything up twice (once on a server, once on an archived separate external), and keep a working copy on your computer.
And currently, 5400 RPM drives are the slowest, but extremely cheap, which makes them popular for long term storage and archive drives. 7200RPM ddrives are significantly faster and not that much more expensive. 15000 RPM drives are primarily used on servers, and are purchased at a premium, and it makes no sense to buy them when you could just get a SSD (solid state drive) for a little more and have better speed, performance, and stability.
I deal with bulk data storage for work (storing PPI), and what tends to be the best practice is when you capture a file on your computer, duplicate one copy to a working directory, and one as the "clean" version to the file server. Then back up your file server weekly, monthly or not at all (depends on whether or not you feel the need to do so). Thus when you cleanse your working directory after it gets too large, you have the files backed up on the fileserver, and possibly on an external backup if you don't trust the longevity of the server.
