Here are my specific recommendations.
Local storage. If you can, keep all your images on internal drives on your main workstation. If you have several networked workstations, use file synchronization software to keep all copies in synch (be careful of two way file synching; done wrong, it can turn a small error into a big one; if possible, do all your editing on one workstation so you don’t expose yourself to this possibility). If you must use external drives, use eSATA or Thunderbolt interfaces. If you want to have local storage on a RAID box, feel free, but it’s not necessary.
Local Backup. Use file synchronization software in backup or one-way mode to copy images automagically from your workstation to an external disk, an external array, a local file server, or a NAS box. If you want to have local backup on a RAID box, feel free, but it’s not necessary.
More Local Backup. Use file synchronization software in backup or one-way mode to copy images automagically from your workstation to another local file server or NAS box. Alternatively, use file synchronization software in backup or one-way mode to copy images from your local backup server to another local file server or NAS box.
Offsite backup. You’re going to be carrying your disks to your safe deposit box or to a friend’s house (if you have a like-minded photographer for a friend, you can keep her backup disks and she can keep yours). You want to be able to quickly remove disk drives from the device that writes them. If you don’t use a RAID, you can buy USB devices that accept bare drives. Hook one or more of them up to your workstation or file server and do backups to them with file synch software in one-way mode. Pull them out occasionally and take them off-site, bringing back the old off-site ones to be rewritten. I’m gun-shy enough to recommend that you have at least two off-site copies of every backup.
You can buy RAID boxes that take bare drives; Drobo is one of the main vendors. I used to use Drobo boxes to writer the off-site backup disks, and then take the whole disk pack to the bank. The Drobo boxes work if you reinstall the disks in any order, but you need to put the drives in the same kind of Drobo box that wrote them in the first place (a regular Drobo can’t read Drobo FS disks, for example), and if your house burns down, it might be hard to find the right Drobo. That’s the reason I’ve gone back to plain disk drives, written in standard formats.