If speed matters then you need to be looking at SSDs rather than hard drives while you'll inevitably pay more for a lot less storage, you'll get blazing-fast transfer speeds and, thanks to their complete lack of moving parts, much better reliability.
Depending on how much you want to spend, you can get a small and inexpensive drive that'll easily hold your most important data, from simple documents through to larger video files, with room to spare, or you could instead invest in a full backup solution that'll hold everything on your main drive so that it's safe and sound if the worst happens.
