Dear Aunt TUAW,
So here's the deal. There are 8-terabyte drives on the market now which, unless you are working for a production company or shooting buckets of RAW, you are never going to use. That's just extra space, which is great!
Except, what is the lifespan of those drives? Will they fail on the average user before they even get close to capacity? If so, what would you recommend for the average user as the best value?
In other words, if I do minor photo and video editing, keep a lot of photos, download a few movies and have a decent iTunes collection, will my need for storage hit 1TB or 2TB before they fail?
Your loving nephew,
Although Uncle Chris is snarking quietly about "640 K should be enough for anybody" jokes, Auntie is going to ignore him and try to do her best to answer seriously, because one's drive space never exceeds one's grasp.
As a rule, Auntie recommends replacing hard drives (especially those used for Time Machine backups) every eighteen months. There is absolutely no science or engineering behind her answer, only a lot of frustrating experience. Of course, you can say that any machine with moving parts is technically prone to failure at any moment...
Eighteen months is about long enough for each new generation of storage to appear on the market and to ensure that fresh storage will meet your ever-growing data needs: it's never just about RAW images. There are home movies, TV shows (hello rapacious EyeTV recordings), audiobooks and more.
The minor photo and video editing that you've described, plus a few movies and a decent iTunes collection can happily live on 1-2 TB for a while, and you'll know when you're about to burst at the data seams. Just make sure the age of your drives and backup system doesn't put your data at risk.