A good backup strategy should minimize single points of failure that can cause complete data loss.
Generally “Backups” are for the active drive attached to your computer, and “archives” are offline storage of older files.
A simple but effective backup strategy is to have two external backup drives (let’s call them “Blue” and “Red”).
The Blue drive is kept off-site. The Red Drive is attached to the computer and you run incremental backups every night. Once a week you swap the drives - take the Red drive to the off-site location and bring back the Blue Drive. The Red and Blue disks should be different brands.
Incremental backups are preferred rather than copying the entire disk. An incremental backup copies only those items that were supposed to have changed. A full disk image copies everything. If there is hidden disk corruption on your master drive, a full disk copy will overwrite the good files on your backup with the corrupted file. An incremental backup won’t copy a file if the OS doesn’t think it has changed.
An offsite and offline copy makes it difficult for malware to delete or encrypt the data. If malware encrypts all your online data, your offsite copy should still be OK.
The Red and Blue disks should be different brands. This reduces the likelihood that a single manufacturing defect will cause both backup drives to fail at the same time.
One backup should always be offsite. A common cause of data loss is someone stealing your computer and the attached drives. Others are a roof leak, knocking over the table, and a fire. An offsite copy helps protect against these hazards.
When swapping backup drives, you should always bring the local disk to the remote site, and then the other remote disk back. You want to avoid the situation where all copies of the data are in the same place.
As to RAID devices, these are helpful, but do not eliminate the need for other backups. A RAID like device can protect you from data loss due to a single hard drive failure, but there are still many hazards that can cause the loss of all data on the RAID device. For example: the failure of the RAID controller, user error overwriting your master file with a low rez web image, someone knocking over the table, a roof leak, or theft.
Obviously, one can get even more carried away. For instance, you can add additional disks to the backup rotation, or make archive copies of your projects on other types of media (perhaps optical media).
When it comes to archiving, I suggest a similar strategy. Keep multiple copies in multiple locations. Every few years, merge your archives onto newer (larger media). The capacity of storage devices is increasing and price is going down. After a few years you should be able to copy multiple backup archives onto a single new storage device.
Archiving has other issues which add an additional challenge. Simply saving a file may not be enough, you may not have software that can read it. I have archived images on PhotoCD discs. I don’t have software that can read these files (Photoshop dropped support long ago). I have digital images from one of the first consumer digital cameras (Apple’s QuickTake 100). I no longer have software than can open these files. Actually, I still have the software, I no longer have a computer that can run the software.
Of course, the above is not the only reasonable strategy. There are many reasonable backup strategies. Depending on how important you data is, you can make a minimal effort, or go way overboard. You have to determine what it’s worth to you to protect your data.
The important concept in any good backup strategy is that it minimizes the chances you will lose your data.