The 3-2-1 rule is a best practice for backup and recovery. It means that when you build out your backup and recovery strategy you should:
1. Keep at least three copies of your data
That includes the original copy and at least two backups.
2. Keep the backed-up data on two different storage types
The chances of having two failures of the same storage type are much better than for two completely different types of storage. Therefore, if you have data stored on an internal hard drive, make sure you have a secondary storage type, such as external or removable storage, or the cloud.
3. Keep at least one copy of the data offsite
Even if you have two copies on two separate storage types but both are stored onsite, a local disaster could wipe out both of them. Keep a third copy in an offsite location, like the cloud.