I'm playing catch up this week after being down with computer issues for a few days last week. My main data drive where my photo libraries are kept started acting up. It wasn't a single catastrophic failure - I just knew the drive was imminently going to go belly up. It had developed frequent slow downs and odd noises. At a little over 3/4 capacity I had plans to replace it anyway this summer after hockey season wrapped up. It decided to retire itself a little early.
This seems like a good time to step on my soapbox and preach about the importance of backups. I frequently hear about people needlessly losing data because they don't have a backup plan in place. Hard drives fail eventually - it's not a matter of if, it's when. I recommend at least two backups of any files you value - one local and one offsite. My backup plan involves a secondary drive that I manually backup to at least weekly or after any photography job. I use a program called Carbon Copy Cloner to do this. Additionally, I subscribe to BackBlaze for my offsite backup. Every night my files are synced to the cloud. Technically I have a third backup of most of my important photos, as I upload them to my web portfolio host, SmugMug. One final measure - my Lightroom databases that manage my photo organization scheme and edits are backed up separately on a weekly basis. This provides a little extra protection in the event of database corruption - it happens.
Speaking of drives, I prefer to keep things simple. I use individual drives rather than RAID setups. I have worked for almost 20 years in storage testing and development and I have to say that I just don't trust any consumer level RAID devices. They just don't go through the rigorous testing that enterprise class stuff goes through. If you do choose to use some sort of RAID, just remember that RAID is redundancy for uptime. It is there to keep you up and running while you replace a failed drive. It is NOT a backup. I also stay away from the low priced consumer external drives that you find in discount stores. I've been using OWC Mercury Elite Pro external drives for a few years and I like them. They use top tier 7200rpm drives and I like their fan-less aluminum enclosures. The whole enclosure acts a heat sink to keep the drive cool. This is important as a lot of failures can be attributed to heat. A weak point in a lot of enclosures I see is the fan. Companies will stick a 10 cent fan in a $200 drive enclosure and when that cheap part blows its bearings your drive is toast. G-Tech makes similar fan-less enclosures. Enclosures like these will be more expensive than the "bargains" you see advertised but they are worth it. Whatever you get, remember that you need to buy at least two for primary and backup.